No problem bud, post away. Tailend Charlie made a 142 conversion so honestly, he may be the one to know. I'm not all too familiar with the history of 131. Not sure if you've ever visited the Tigers in Focus site, but they have big discussions going on. Here's a link just in case. They've posted tons of pic's there where I've tried to help identify this for you, but it only managed to confuse me. For instance, the various pic's that are shown...there are so many changes, from the painted numbers, mounted tracks on the glacis, varied exhaust covers from the restored version to the version that's on Tailend Charlies's version, modified head lamps, missing front fender to repaired front fender.......so many differences with no accurate timeline in those pics.

But if you are aware of most of the changes to a specific timeline like the original identification number vs the altered id number, you may be able to tell when these modifications occurred.
http://www.tiif.de/search.php?search...d9cbfb8dc328d8

Sorry I couldn't be more help.