Great photo ! Is that you in command ?
OK, so its got the chrome tank panels with winged round tank badge, a tin primary cover,
a painted front mudguard (fender) and a full width alloy front wheel.
And a twin, obviously. All pointing to somewhere in the mid-late 1950s.
I'm going to plump for a 1957 Model G11 Matchy, a 600cc twin.
Click for a bigger pic
These sold in large numbers, particularly in the USA.
(more there, darned if I can link them )
Someone who really knows their models may be better able to totally pick if its 500cc, or 550cc though,
all very similar and similar era. Apart from some having a slightly fragile crankshaft setup, a good seller,
and Matchless were renowned for the quality of their finish. USA models often had more chrome.
P.S. The engine number will often include the Model type AND year, so if your cousin could confirm that
you would have it 100% nailed down. What was exported will often vary slightly from what was in the British
brochures, which can lead to some variations in identification.