Welcome to the Thompson Forum. Your Auto-Ordnance Bridgeport (AOB) 1928AC was at one time owned by a Board member. I believe his Board moniker is TNKen. If you search the Forum you may find some pictures of your Thompson. I believe the original owner was the Mobile, Alabama Police Department.
May I suggest you purchase American Thunder, Third Edition, by Board Member Frank Iannamico to learn more about this Thompson variation. Actually, you should have had this book, or Tracie Hill's, The Ultimate Thompson Book, in your library before seriously shopping for a Thompson gun. Your Thompson gun is not a military or GI Thompson. It is basically a parts gun sold commercially to law enforcement organizations by Auto-Ordnance Corporation, later to become Maguire Industries, after the production of the Model of 1928 Thompson gun ended. Known years of sales were between 1943 and 1946. The removal of the receiver nomenclature markings are pretty standard for this variation. There are many threads on the Forum about the 1928AC variation. Unfortunately, there is much misinformation on this variation, but Frank's book will get you pointed in the right direction.
May I also suggest you join The American Thompson Association and/or The Thompson Collectors Association and attend one of the yearly Shows and Shoots.