77.303 Mainspring from R & M Imports
September 15, 2007 . by BillIn previous posts, I mentioned the R & M No. 77.303 mainspring as being about .014 inch thick. These thin springs can be identified by having the number 37 on the loop end (which means 0.37 millimeters which equals 0.0146 inch).
I received a new batch of these today, and the number on the loop end is 42 (0.42 millimeters which equals 0.0165 inch). I measured one spring to be 0.0160 inches thick.