It's basically a Colt Service Ace in .22LR, and in excellent condition can be worth well North of $1000.
FWIW, In the 1960's the province of Buenos Aires (Argentina) police replaced its service caliber .45 pistols (Colt, Ballester-Molina and Sistema Colt) with Browning Hi Powers in caliber 9 mm Para.
Some of the caliber .45 ACP pistols were afterwards transfered by the Buenos Aires police to other Argentine provincial police and the remaining Ballester-Molina and Sistema Colt were converted to caliber .22 LR, purportedly to be used for training.
Your evidently looking at one of the Sistema Colt conversions.
Two companies undertook the conversions, Establecimientos Venturini SA, and Industrias Marcati SCA.
After being converted to caliber .22 many of the pistols were stamped with the name of the company that performed the conversion, and the new caliber.
The Marcati and the Venturini conversions differ in the way they were implemented and a quick inspection can reveal if it is a Marcati or a Venturini converted pistol.
The main mechanical differences between these conversions are:
*The Marcati conversion has an ejector permanently attached to the frame, a hole and a rolled pin can be seen on the side of the slide and the magazines look like the magazine of a caliber .22 rifles but with a spacer attached to its back.
*The pistols converted by Venturini do not have an ejector attached to the pistol. The rear part of the left lip of the magazine is raised and doubles as the ejector, there are no holes or cross pins on the slide and the magazine has a double wall. The outer wall of the magazine duplicates the external dimensions of the pistol's magazine well. Many Venturini magazines were marked with the serial number of the matching pistols using an electro pencil.