I believe that this is a reference 5191 from circa 1944. Movement is likely a caliber 10L/10.68Z. "22570" is the order number. Longines can confirm this information if you send an email to them with pictures of the watch and the seven digit serial number, which is on the movement.
Collectors would call this a "sei tacche" (six notches, in Italian). This refers to the design of the case-back. These are desirable watches in good, original condition, as this one is in. The case does not look to have been polished and the radium numerals on the dial have aged nicely. Furthermore, black dials are particularly sought-after.
As others have stated, make certain that the dial is not touched. This is a fantastic piece!
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