What are these calipers original to?

Insufficient information. Need to see fluid inlet at a minimum. Also a top view. What are they on presently? Piston size would be very useful also.

Piston looks to be 34mm.
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[quote name='SierraNevada' date='Sep 5 2018, 09:31 PM' post='2645930']
Piston looks to be 34mm.
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Calipers are off the rear of a vented rotor 914.

Nope not the rear of a 914 at least not the stock. Only vented calipers on the rear of the 914 was for the 914-6 GT cars. they where much like the 914-6 caliper with hand brake in the body, and had the spacers and longer bolts allowing you to run a vented rear motor. These Calipers where also used on the Ferrari 208 - 308 cars. Only issue is the bleeder valves are on the bottom when installed on a 914 trailing arm. PMB can fix that issue.

With the limited information and odd angle pictures, I would say 911 M caliper, that got shaped to the rear. Not an uncommon swap, you do loose the parking brake unless the 911 bits have been shaped also. HTH

That fluid inlet is 911 or 912 rear. You will need to remove the dust seal to get the actual piston diameter but I would guess that it is the 38mm version. So this would the normal 911 rear M caliper for vented rotors from 1969 until the 42mm version in the SC years. The hollow piston was the late version. This would be equivalent to the 914/6 caliper in piston size, and the 914/6 GT with vented rotors. You would need a 911 parking brake conversion for safety & road legal purposes.