1/2" chain
Measure 12" of the stuff,if it is 1/2" pitch chain,you will have 12 drive links.If it is .404,you will have almost 15[ 14.85 to be exact]3/8"would be 16.It has been obsolete,for years.Old gear drives used it.It takes some power to run it,but takes a much deeper "bite"than .404,clears the chips better.Saw racers,just love it[I do too].AS far as a bar,I have only seen it ran on either a hard nose bar,or and open roller bar,and am not aware of any sprocket nose bar,although there may be some.It would not be that hard to modify an old bar, of these types from,say an old Mac,or Homie to fit the Stihl.The only driver thickness I have ever seen,is .063,although I think it may have been made in .058.The only problem,I could see,would be lengthenig,or shortening the chain,for lack of the tie strap rivets.These could also be fabricated.This is only an opinion,but if it were my saw,I would leave it as 1/2".Although it is hard to find,it can still be found.I have a 56 drive length loopof 1/2",as well as 20 other new chains,for less than $40.coming soon[ on e-bay, you have to read the fine print]Also it uses a 1/4" file,to sharpen,readily available from McMaster-Carr as well as other suppliers. Good luck Al