Researcher, the Folsom guns appear in two separate serial no. ranges. One range is 1F-17,000F. The other is in the 201,000 range. I am not sure of the timing or significance of either range.

As to the letter, William M. Locke was the owner of one of the finest weapons collections. A revered collector good enough to have Frank Sellers write a book on Locke's guns etc. I am not sure if others might have done the same.

I'd suggest that the letter and gun must have been done as a friendly joke. No other explanation comes to mind.

http://www.bakercollectors.com/