Spiwak is known for his terrible pictures. However, that seems to work for his business model. The supply of machine guns registered for civilian ownership is finite so potential buyers with the bug have a limited selection at any one time without a lot of patience. Given today's prices, most informed buyers would go see the product if they see something that is of interest at a reasonable price. All that said, you acquired a known Thompson variation that will make a great shooter Thompson. Shooting your AOB 1928AC should not devalue it in the least. Knowing the police department that purchased your 1928AC is also a plus. When you obtain possession, I would suggest you file a Freedom of Information Act (FOIA) request with the BATF to learn when your Thompson was originally sold. Frank's book has an unredacted copy of an original IRS Form 5 for a 1928AC on Page 150. BATF will only provide a redacted copy but this is the start of compiling the provenance for your 1928AC. During your wait you can read Frank's book and research the Board about FOIA requests.
Here is the link to join TATA and/or TCA. Both organizations hold shows & shoots in Ohio every year. Usually around the first of August (TATA) and the middle of September (TCA). Both organizations are great and the events are a lot of fun. Tracie Hill's collection is on display at the TATA Show.