Hamiltonparts dot com has a parts list for the Hamilton 2974B, it lists a couple of options for strength. Base number for the mainspring is 60061 for standard strength. If you go to the website, there is a section on these watches with a parts listing and diagram, both are downloadable.
Thanks for that, Don. Unfortunately, I'm not quite sure what those part numbers represent. They don't line up with any of Hamilton's 'old' or 'new' part numbers. I suspect they are special military part numbers.