It is my understanding you recently posted S-23551 on gunbroker.com. Several forum members asked me to review the photographs of S-23551 you posted on gunbroker.com and offer an opinion on originality and value. I stand by my above post and can offer more insight to this early Savage Thompson. See:
This forum has an amazing amount of preserved history. Of course, I have been capturing all the information I can find about the early Savage Thompson guns for many years, hence forum members reaching out to me for my thoughts on your S-23551. I only reviewed the pictures you posted on gunbroker.com. If any part of my above post in inaccurate, please let me know. I also respect the opinions of David A., Darren and all RKI's.
The fact S-23551 was owned by a family for 25 plus years is interesting. The fact they owned a Colt Thompson and other rare Thompson items is also interesting. I am not sure what this means when evaluating S-23551 for originality or more importantly, value. That the manufacturer is listed as "Colt" is also not surprising. The IRS and ATF used to accept anything on these registration forms years ago. I dare say "Kangaroo" would not have raised an eyebrow at the IRS when S-23551 was born on the Registry. Of course, it means nothing in judging originality and value.
The frame attached to S-23551 is not from the Savage factory. It is from the Auto-Ordnance Bridgeport (AOB) factory. Regardless, it has been renumbered and is not original to the receiver. Since you are just a self-professed Thompson owner, not a collector and not knowledgeable in this area, please do not take offense when someone with some expertise in this area explains what you own. Let me continue, for your edification and those on the forum that purchase my books and ask my opinion on these very expensive toys.
I noticed what appeared to be the "NAC" markings on the receiver and frame before searching this forum for information on S-23551. Since S-23551 was originally advertised for sale as S-23551 NAC in 2013 (along with Colt's NO 5086, which I assume you also obtained from the estate since you recently offered it for sale) I am curious as to who had S-23551 NAC refinished. As you can see from the below pictures you posted and I cropped and changed the lighting, the letters "NAC" were definitely on the frame and receiver as a serial number suffix in 2013.
The Numrich Arms or NAC Thompson guns is another area I have studied for many years and can say without reservation, the NAC markings on any machine gun do not add value to the product. If I were to guess, I would think S-23551 was a 1960s import gun, likely from Interarms, and acquired by Numrich Arms. A Freedom of Information Act (FOIA) request would answer many questions regarding the past history and number of owners.
S-23551 is probably a great shooter grade Thompson gun. However, it is not something that will appreciate in value like an early Savage Thompson in original condition. The original title of the thread said something to the effect S-23551 was something special. Since I had already answered a few inquires, I decided to post a few comments on the forum for the general membership. Each Thompson gun presents a learning opportunity, S-23551 included. I encourage everyone to comment.
I do not consider S-23551 a "POS" but will gladly take it off your hands if you are inclined to give it away. I do like those British proof marks.