buckstix,

I been studying William Powell & Son numbers for some time. Powell made a number of W. Richards guns. From what I've seen W. Richards numbers are some what chaotic. Here are some Powell "lifter"guns that were made for W.Richards. The Powell serial number is in parenthesis. No.6139 (5965) 1875, No.6140 (6074) 1875, No.6141 (6075) 1875, No.6142 (6076) 1875 and No.6763 (6149) 1877. An example of the chaos is No.2269 (4995) 1884.
My overall point is that only the factory serial number records will allow you to absolutely establish the date. You should also consider the possibility that the serial number is from Henry.