We often pick apart cars for sale to help those less knowledgeable about these cars know what they are looking at. Not all that glitters is gold.
The car in question is very nice! It would look great anywhere a model T belongs! I would say it should sell in excess of $15,000. It should not get close to $20K because of the wrong engine! It does not technically qualify for the HCCA national club. Although the dirty little secret is that quite a few cars on their tours also do not technically qualify. The rules are there, and for good reasons. They are not always strictly enforced. Some people do get upset about that.
HCCA Regional Groups are allowed to extend the qualification years and rules to fit local needs. Most Regional Groups would happily allow this car on their local tours. However, some Regional Groups (including the one I belong to!) take a tougher stand with those rules. They may, or likely won't, allow this car unless one is actively working to replace the engine with a correct era engine! They may or may not allow it on a tour as a guest.
Whether the car in question ever was a "real" 1915? At this point is anybody's guess. There are a bunch of good photos of the car on BAT. Several do show a fair number of things wrong on the car. Certain specific details curiously do not show up in the many photos.
The funny thing I noticed in the photos? The rear wheels are wood felloe non-demountable wheels whereas the fronts are later steel felloe demountable wheels! Must be a story behind that?
If someone is interested in looking at the listing?
This might help;
https://bringatrailer.com/listing/1915-ford-model-t-12/