It's the Colt "I" 41 frame with fixed sight they used for the Official Police, Officer's Model Match ( adjustable sights ) revolver, Colt 357 ( a. s. ), the earlier Trooper ( a. s. ) and the Python. The Trooper Mk 111 and subsequent Mk V were a later design that I believe used metal injection molding ( sintered parts ) IIRC
The Police Positive Special was a stretched frame and cylinder version of the "D" frame to hold the longer 38 special cartridge. The PPS, the Detective Special ( snubby steel frame ) the Cobra, and the Agent ( aluminum frame ) were all fixed sight, but the Diamondback was the junior version of the Python.
Except for the Mk III and Mk V they were all sweet... OK, the Mk III and Mk V too, by todays standards.
Anyway if it's still there in a month at $300 I'll offer them something.