Do the firing pin springs interchange between a 639 and later models?

yes... S&W part numbers are different across the models, but it's essentially the same spring.

Wolfe Springs also lists just one firing pin spring part number (26108) for their spring, for use in all of these models:

39, 439, 539, 639, 3904, 3906, 3913, 3914, 3944, 3946, 3953, 3954,
59, 459, 559, 659, 5903, 5904, 5906, 5923, 5924, 5926, 5943, 5944,
410, 411, 4003, 4004, 4006,4026,4043,4044, 4046,
910, 915

__________________
Conrad
SWCA #1830 SWHF #222

OK...thanks.