Hi folks, I measured up the mainspring in a Waltham model 1894 12s and came up with (in mm)  2.10 x 0.14 x 490 or so mm, hole end, with 15.5 barrel ID.  For a few dollars at Cousins, I can get a 2.10 x 0.15 x 480, 15.0 ID, TR end. Otto Frei has a factory mainspring for this watch that they are saying is 2.15 x 0.16 x 19 1/4 in (488mm) with end hole, for about 4 times the cost. One thing I'm sure of is that I might need a better quality set of calipers . With the variability in sec