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<Click>
(A:TRANSPONDER1 CODE, enum) (>L:XpndrStdbyTemp, enum)
'0x' (L:XpndrStdby, enum) scat (>K:XPNDR_SET)
(L:XpndrStdbyTemp, enum) (>L:XpndrStdby, enum)
</Click>


Lesse... it's actually (A:TRANSPONDER CODE:index) and 1 is the only supported index and it's a BCO16 not an enum. While in XML all numbers are just numbers... for code readability when you have to come back to it in 5 years as well as for anyone else trying to figure out what you did... use the correct units.
Other than that... yes, you can use an L:Var to mimic having two codes available.
<Click>
(A:TRANSPONDER CODE:1, Bco16) (>L:XpndrStdbyTemp, Bco16)
(L:XpndrStdby, Bco16) (>K:XPNDR_SET)
(L:XpndrStdbyTemp, Bco16) (>L:XpndrStdby, Bco16)
</Click>



<Gauge Name="Xpndr" Version="1.0">
<Image Name="Xpndr_Background.bmp" />
<Element>
<Position X="64" Y="14" />
<Text X="360" Y="28" Length="18" Fixed="Yes" Font="Quartz" Adjust="Center" VerticalAdjust="Center" Color="%('#FFFF00' '#FF8000' (L:Xpndr Ident,Bool) ?)" Bright="Yes">
<String>%((A:TRANSPONDER CODE:1,Number))%!04d!</String>
<Failures>
<SYSTEM_ELECTRICAL_PANELS Action="" />
<SYSTEM_ELECTRICAL_AVIONICS Action="" />
</Failures>
</Text>
</Element>
<!-- Check for button pressed -->
<Element>
<Select>
<Value>(E:LOCAL TIME,Seconds) (L:Timer_01,Seconds) < if{ 1 (>L:Button Pressed,Bool) }</Value>
</Select>
</Element>
<!-- x000 -->
<Element>
<Select>
<Value>(L:Button Pressed,Bool) (L:Xpndr Digit,Number) 0 == && if{ (A:TRANSPONDER1 CODE, BCO16) 4095 & (* 0FFFx *) s0 (L:Xpndr Button,BCO16) 12 << l0 | (>K:XPNDR_SET) }</Value>
</Select>
</Element>
<!-- 0x00 -->
<Element>
<Select>
<Value>(L:Button Pressed,Bool) (L:Xpndr Digit,Number) 1 == && if{ (A:TRANSPONDER1 CODE, BCO16) 61695 & (* F0FFx *) s0 (L:Xpndr Button,BCO16) 8 << l0 | (>K:XPNDR_SET) }</Value>
</Select>
</Element>
<!-- 00x0 -->
<Element>
<Select>
<Value>(L:Button Pressed,Bool) (L:Xpndr Digit,Number) 2 == && if{ (A:TRANSPONDER1 CODE, BCO16) 65295 & (* FF0Fx *) s0 (L:Xpndr Button,BCO16) 4 << l0 | (>K:XPNDR_SET) }</Value>
</Select>
</Element>
<!-- 000x -->
<Element>
<Select>
<Value>(L:Button Pressed,Bool) (L:Xpndr Digit,Number) 3 == && if{ (A:TRANSPONDER1 CODE, BCO16) 65520 & (* FFF0x *) s0 (L:Xpndr Button,BCO16) l0 | (>K:XPNDR_SET) }</Value>
</Select>
</Element>
<!-- Digit Loop, reset timer & button -->
<Element>
<Select>
<Value>(L:Button Pressed,Bool) if{ (L:Xpndr Digit,Number) 1 + 4 % (>L:Xpndr Digit,Number) (E:LOCAL TIME,Seconds) (>L:Timer_01,Seconds) 0 (>L:Button Pressed,Bool) }</Value>
</Select>
</Element>
<!-- Reset Ident -->
<Element>
<Select>
<Value>(E:LOCAL TIME,Seconds) (L:Timer_02,Seconds) > if{ 0 (>L:Xpndr Ident,Bool) }</Value>
</Select>
</Element>
<Mouse>
<Tooltip ID="TOOLTIPTEXT_TRANSPONDER_ID" />
<!-- VFR Button -->
<Area Top="62" Bottom="74" Left="226" Right="246">
<Tooltip>Set Transponder to 1200</Tooltip>
<Cursor Type="Hand"/>
<Click>4608 (>K:XPNDR_SET)</Click>
</Area>
<!-- 1000 Digit -->
<Area Top="10" Bottom="27" Left="65" Right="80">
<Cursor Type="UpArrow"/>
<Click Repeat="Yes">(>K:XPNDR_1000_INC)</Click>
</Area>
<Area Top="28" Bottom="45" Left="65" Right="80">
<Cursor Type="DownArrow"/>
<Click Repeat="Yes">(>K:XPNDR_1000_DEC)</Click>
</Area>
<!-- 100 Digit -->
<Area Top="10" Bottom="27" Left="85" Right="100">
<Cursor Type="UpArrow"/>
<Click Repeat="Yes">(>K:XPNDR_100_INC)</Click>
</Area>
<Area Top="28" Bottom="45" Left="85" Right="100">
<Cursor Type="DownArrow"/>
<Click Repeat="Yes">(>K:XPNDR_100_DEC)</Click>
</Area>
<!-- 10 Digit -->
<Area Top="10" Bottom="27" Left="105" Right="120">
<Cursor Type="UpArrow"/>
<Click Repeat="Yes">(>K:XPNDR_10_INC)</Click>
</Area>
<Area Top="28" Bottom="45" Left="105" Right="120">
<Cursor Type="DownArrow"/>
<Click Repeat="Yes">(>K:XPNDR_10_DEC)</Click>
</Area>
<!-- 1 Digit -->
<Area Top="10" Bottom="27" Left="125" Right="140">
<Cursor Type="UpArrow"/>
<Click Repeat="Yes">(>K:XPNDR_1_INC)</Click>
</Area>
<Area Top="28" Bottom="45" Left="125" Right="140">
<Cursor Type="DownArrow"/>
<Click Repeat="Yes">(>K:XPNDR_1_DEC)</Click>
</Area>
<!-- Button 0 -->
<Area Top="60" Bottom="72" Left="10" Right="28">
<Cursor Type="Hand"/>
<Click>0 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 1 -->
<Area Top="60" Bottom="72" Left="33" Right="50">
<Cursor Type="Hand"/>
<Click>1 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 2 -->
<Area Top="60" Bottom="72" Left="55" Right="72">
<Cursor Type="Hand"/>
<Click>2 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 3 -->
<Area Top="60" Bottom="72" Left="77" Right="94">
<Cursor Type="Hand"/>
<Click>3 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 4 -->
<Area Top="60" Bottom="72" Left="99" Right="116">
<Cursor Type="Hand"/>
<Click>4 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 5 -->
<Area Top="60" Bottom="72" Left="121" Right="138">
<Cursor Type="Hand"/>
<Click>5 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 6 -->
<Area Top="60" Bottom="72" Left="143" Right="160">
<Cursor Type="Hand"/>
<Click>6 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button 7 -->
<Area Top="60" Bottom="72" Left="165" Right="182">
<Cursor Type="Hand"/>
<Click>7 (>L:Xpndr Button,BCO16) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_01,Seconds)</Click>
</Area>
<!-- Button Clear -->
<Area Top="60" Bottom="72" Left="189" Right="208">
<Cursor Type="Hand"/>
<Click>0 (>L:Xpndr Digit,Number)</Click>
</Area>
<!-- Button Ident -->
<Area Top="15" Bottom="30" Left="5" Right="25">
<Tooltip>Ident</Tooltip>
<Cursor Type="Hand"/>
<Click>1 (>L:Xpndr Ident,Bool) (E:LOCAL TIME,Seconds) 3 + (>L:Timer_02,Seconds)</Click>
</Area>
<!-- Button Switch -->
<Area Top="34" Bottom="45" Left="5" Right="25">
<Tooltip>Switch Frequencies</Tooltip>
<Cursor Type="Hand"/>
<Click>(A:TRANSPONDER CODE:1,Number) (>L:XpndrStdbySwap,Enum) '0x' (L:XpndrStdby,Enum) scat (>K:XPNDR_SET) (L:XpndrStdbySwap,Enum) (>L:XpndrStdby,Enum)</Click>
</Area>
</Mouse>
</Gauge>