The number on the caliper is the casting number. When it is machined (with different piston sizes for example) it got a new number.
-That's how it used to be.
The finished part number was usually stamped on, but with age, it might not be visible any more. Look on the flat faces: sides of the mounting lugs, end faces etc.
-And yes Automotive Products is defunct.
But AP Racing has been in business since mid '60s (ish), and still going strong.
Brakes had 'Lockheed' on them, and clutches had 'Borg and Beck', to advertise the production-vehicle side of the business
Going off-topic somewhat, AP also designed and built the auto gearbox for the Mini. -The engineers reckoned there was room to have made it a 5-speed, but it was already a 'sensation' to have a 4-speed auto! (and in such a small car!)