[Heathkit] Need HM-2103 help

[Tom NØJMY - AAR7FV]

I picked up a Heathkit HM-2103 dummy-load/wattmeter at a hamfest a 
couple years ago, and just now got around to looking at it.  This is the 
one whose face is about the same dimensions as a HM-102 but is about a 
foot deep with a perforated rear section to house the air-cooled dummy 
load resistor.

Someone added a spst switch on the back that appears to cut in a 
pcb-mount type pot that is between the original factory pot on the board 
and a point marked "H".  Cutting it in gives a lower power reading.

I have no manual or schematic, neither being on BAMA.  Can anybody tell 
me what the purpose of this mod is or might be?  Also, does anyone have 
a manual or schematic or copies of same that I could acquire?

Thanks in advance,
Tom