I did some searches but was not able to find what type of mainspring goes into a Hamilton 2974B comparing watch (SN:2K11054). Thanks.
Dan

Dan,
I’m not positive but pretty sure it is the same size mainspring as the 992B. There are three different strengths given for the Dynavar mainspring 350 (minimum), 370 (standard), and 390 (maximum). Usually watches with fewer jewels require stronger mainsprings so it’s best to measure the spring currently in the watch and replace it with one of the same strength. The thickness of these springs are .155, .160, and .164 for maximum strength all in MM. The length of the spring should be 29.5 inches.

RR