Toby, I am surprised that you have not noticed bar in wood conversions. I , for the last several years have enjoyed pinfire guns and have shot them with some success. If one looks closely like I have, one can see many Pinfire s converted to center fire. Westley Richards, I have seen a dozen or more, did a fine job of converting to center fire. Some are quite magnificently done. A friend has a Purdey center fire conversion that went through at least muzzle loader and pinfire configuration before becoming a center fire. Actually, I seem to remember that it may have started as a flintlock, but it was years ago, and I’m not sure. A friend enjoys William Powells and their conversions to center fire are not uncommon. Many American conversions were made, too. I have a fine Genez that used Williams and Powell locks, barrels, etc to convert to pinfire. Many makers on all continents advertised that they did conversion work.
I have no doubt you are correct that they are more common than one might think. I was assuming you were talking ML to CF which is considerably less common than PF to CF. I have no doubt that if you are looking for them, PF to CF will pop up a lot. However, as I only have an interest in guns that are proofed for, or reproof-able for, modern nitro loads, anything with pinfire barrels are quickly passed by. I have reproofed a couple successfully in the past but with the stricter proof tests nowadays, it seems a pretty dodgy proposition. As regards the numbers of original CF BiW against PF - CF BiW, I would say that I have probably seen a 10 -1 ratio over the last 20 years. As to the chance of misidentifying a PF - CF conversion as an original CF, I feel there are so many indicators of PF origins that I would be surprised if I wasn't suspicious if not total sure in 99% of cases. Of course the whole gun can be reworked into the 'modern' idiom but you can't 'invent' the chamber walls of a CF from the slim lines of a PF. Only the very earliest CF had such light chamber walls and most were probably destroyed when nitro powder came along.