I have several FBI 1076s with recall punch marks. Keep in mind that any subsequent owner of the pistol could have sent it in for the recall inspection. The inverted T is indeed a mystery. Many, but not all, of my FBI pistols have it. There are several other letters and numbers stamped on the pistol. I believe at least one of them identifies the gunsmith assembling the pistol. I haven't found anyone that can decipher the various markings on the pistol. Even the inverted T is in question. In many cases it appears to be the number 1. Note also that your barrel has one, if not two, letters stamped on it. The most common is an O.