Sorta Confused Maybe
It's entirely true, Neither Colt, nor ASM, nor Uberti ever built an 1861 Richards - Mason conversion. Just didn't happen. However:
Based on previous, it may well be Pappy is actually looking for an 1861 Richards Conversion. Now it gets a mite "gray." Colt never built 1861 Richards Conversions. Uberti never built 1861 Richards Conversions. Armi San Marco did. Sorta. Almost, Real Close. What ASM called their "1860" wasn't. If you look close, no stepped cylinder. The ONLY thing distinguishing the gun as an 1860 are the 1860 Army Pattern Grips. Straight unstepped frame. unstepped cylinder, and an 1860 pattern barrel assembly. Pull the 1860 Army Grips, swap in a set of 1851 Grips and you have an 1861. Or, at least as close as you're ever going to get in this day and age.
Just my take.