Due to PM's we believe it's most likely not an error in the letter. According to Dougans list in his book, the gun in question was shipped with a the short bbl.
To further help collectors,,, here is a little story of one of my guns.
I attended an auction several years ago & there was an OM 45 convertible, boxed, with 2 sets of grips. A set of aftermarket Sambar stags, and the original grips. There was paperwork in the box etc. Knowing that Om convertibles were desirable, and adding the stags,,, I bid enough to win the auction. I sold the stags to help offset my expenses. But the gun was an enigma. In some of the paperwork, it has a work order/receipt where the gun had been returned to Ruger for them to add the 45 acp cylinder. That made it a factory done convertible. When I sent for my letter, (this was before they even charged for them,) it came back as NOT a convertible. A few phone calls & lots of patience etc,,, along with the details provided by me & the work order, did not help the wonderful girls who TRIED to find info on my gun. It just will not letter as a convertible. The letter can only state how the gun was originally shipped. They could not find any records that showed it as a returned gun for additional work.
My point is that it's possible that this gun above was returned to the factory for the addition of a different bbl. Ruger can & will often do such modifications,,, IF,,, IF,,, they already make that model. I'd suspect in this case,, he will never get a true, factory letter on how it is now.