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quanthe toy town stamp to me is a back stamp and is not an overprint since it does not make references to anything, like the e94 overprints or e98 Old Puts. They are found on various backings of different e and t issues and does not carry any extra premium to me. I would not pay more, and consider them a deterrent like the other initial back stamp of previous owners. Some like Richard think they are unique and collect them, maybe he's the genius and I'm the idiot, but...
PSA have always given TT the MK marking, and SGC had always marked a TT card down at least 2 grades from what I saw. An e97 Kelley psa4mk with the TT stamp was graded sgc30 for example. I don't think SGC would change their mind midstream and now grade these cards as sgc50s with the TT labeling, that would be extremely careless.