A letter is prudent, but even without one, a gun in the tight, well known serial number range that also has a mag cylinder with it has a high probability of being legit. Of some 1000 convertibles made, about 300 mag cylinders were returned, so they are not floating around in junk drawers, at gun shows and on eBay like Single Six mag cylinders.
I've bought 3 of these, and sold one of them - twice. That gun didn't have the box or shipper and the buyer later upgraded it to a gun with box/shipper, so he sold it back to me. The first time I sold it was for $1250 + shipping (here on RF), the second time was $1382 + shipping (GunBroker). Of course the GB sale cost me about 60 bucks in fees.
The other two I have cost me $836 (box, no shipper) in 2012, and $1744 (box and shipper) in 2017.
I have seen them sell for over $2K and have a note that one with no shipper sold on GB in 2015 for $2026. It just depends whether two people who have to have it are looking at the time you list it.
WTS: Original Bearcat .22/ .22 Mag convertible. Going from memory here... I recall ....1000 made in early '90's. Ruger recalled the .22Mag Cylinders. Claimed timing was incorrect. I think the lawyers were working OT. As many as 600 - 700 cylinders were sent to Ruger. Customers nev...