Hi,
When an R frame looks like a K frame, there's a sure way to know if it's an R rather than a std stripped K frame:
Look at the horizontal extensions for the exhaust bracket on the tomahawk: They are not left as cast on the KR but milled down before the R frame is assembled/silver soldered to give a dead flat surface for the KR rigid rear frame bottom run to bolt on...
The early KR frame was moded in 1958 so that the XLR engine can fit in...
I also saw once on ebay a 57 "one year only" race frame with a unique goose neck and "no 2 stages" down tubes, might have photos somewhere...
Patrick