I recently picked up a P220ST in Gunbroker, as marked on the slide, but the frame is aluminum. The frame, barrel and slide serial numbers match and without proof marks.

I thought all P220STs were all stainless and didn't ask the seller about the frame material.

Were most P220STs all stainless?

Just like their SIG US counterparts, the Germans can assemble anything into a complete gun. Yours has the "ST" (stainless) slide on an aluminum frame. Basically, just a regular Nitron P220 that was made and proofed in Germany. Are you sure there are no proof marks on the gun?

Is this the one?

Yes, that's the one.

The proof marks are right there next to the serial numbers on the frame and slide. The barrel proof mark, together with the 2-letter date code, will be found on the hidden side of the barrel hood.