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EDIT: I am having trouble adding the pictures, they wont add for some reason. Is there a size limit?
The Grand Prize was a mid/lower tier watch, mid 1960s.
Yours looks like a US market watch with an aftermarket 14K gold bracelet.
Flagship/Conquest
Admiral/Grand Prize
Apart from the name, there was not much different in the higher-end "Conquest" or "Flagship" lines compared with your "Grand Prize." It has an "Admiral" case, which required a special type of wrench to open at the time it was made (I think other tools can be used by a watchmaker now). It's a one-piece, front-loading case that was designed to have some water resistance.
Indeed, most Longines watches shared calibres between families, which was different from, say, Omega's policy of keeping the first class chronometer movements (with a mention on the certificate for "especially good results") for Constellations, and second class chronometer movements for Seamaster chronometers (pass but without special mention).
Even though Longines shared movements between families, very rough equivalents between Longines watches and their competition can be made:
Longines Flagship = Omega 30 mm = Rolex Oyster Precision
Longines Flagship Automatic/Conquest = Omega Constellation = Rolex Datejust
Longines Admiral = Omega Seamaster = Rolex Oyster Perpetual
Grand Prize = Omega DeVille = Rolex Oyster Perpetual
Note there's a lot of overlap so the guide is best taken with a large handful of salt.
It is really quite perplexing to imagine how Longines came up with their hierarchy.
It becomes simpler if you look at them as all sharing components such as movements and sometimes cases of the same quality, and their name being more of a nod to their competition from other brands rather than a Longines brand hierarchy - Conquests competing with Constellations, Admirals competing with Seamasters, etc... A Grand Prize would be just as well made as a Flagship, just different intended markets.
