The 300 Savage is a shortened 30-'06 with a neck that is just about the shortest among commercial cartridges. The 250-3000 Savage is a necked down 300 Savage. The 22-250 is the 250 necked down. The 250 AI was one of Ackley's personal favorites and is a cartridge that, arguably, benefits more than any other from being "Improved". The 6.5x22-250 AI would be a cartridge shooting bullets that are .007" larger than the 250 AI.
The 308 is a lengthened 300 Savage, with a longer neck, but not as big as a 30-'06. The 6.5 Creedmoor is a slightly
shortened 308 case, necked down to shoot 6.5 bullets. I think I need a flow-chart!!
As Kaytod astutely points out, this odd wildcat will be virtually identical to the 6.5 Creedmoor or the Lapua. Throw in the slightly larger 260 Remington and 6.5x284...and you can see this is already a pretty crowded neighborhood. As convoluted as it was to get there, it's probably a good round.
A few thoughts:
1) Please take a fired, unsized case, with the primer still in it, and determine the case capacity. That will be essential in determining what you're working with and what loads are reasonable.
2) If the seller is telling you it likes 100gr bullets best, I would be asking what rate-of-twist the barrel is rifled for, because that will let you calculate for yourself whether or not it will work better with the heavier target bullets in 6.5mm.
3) The most important thing is to understand what you have and then enjoy it for what it is. I would not have chosen a Model 7 for a custom build like this, but if the thing shoots, that's what matters.
Be sure to get back to us with case capacity and ROT so we can give you some ideas on starting loads with various bullet weights. If the capacity proves to be very similar to the Lapua, and I fail to see how it could be otherwise, the better powder choices are likely to be one of the 4350's or Reloder 19. The working pressure of this 'cat is likely to be ~53,000psi, based on the parent case and the very similar 250 AI.
I sure hope to hear more about this one.