The handset doesn't really match the case, in my opinion, either, though it does match the dial, suggesting that the dial and handset have possibly been re-cased into a later case. TBH, I am not sure what it signifies? Case style, perhaps? Case material? My 1956 stainless steel Conquest has '719' in that location but none of the other four - non-Conquest - Longines that I have from the 1950's/early 1960's have it). The case looks genuine (the three digit number on the underside of the 11 o'clock lug is a feature of the Longines All-Guard and later Conquest watches from that era. Black dial watches from the 1950's are rare - though not unheard of, so I could be wrong - and the white print ('Longines' for example) in your photos looks quite uneven and rough compared to an original Longines dial from the time period.

I suspect that the original dial was white or silver or cream.