Is that off chainsawcollectorsse forum?
Copied stihl#1 post
" I found a guy still working at the plant that remembered something about the R meaning it was an experiment to try some with a bigger piston for more power. So I pulled the muffler on an 015 in the museum and measured the bore and it is 38 mm like the IPL says it should be.
SO then I measure the bore on the R model and sure enough it is 1 mm bigger at 39 mm. No tech bulletins, no part numbers for the bigger cylinder and piston, just a small trial run so that makes these pretty rare in my book. I have only ever seen one other, last year, and that saw came from a guy who retired from STIHL after 40+ years so he got it off the assembly line when it was new.
The other really weird thing about this saw is that it has this funny bracket looking part cast into the front cover:"