Interesting bike. Frame, tank, transmission and clutch petal all seem older than an engine with a date of 1-53 as on the engine plate.
In May of 2014, RandyS posted pics of his bike which has the same contract number as your bike and a 53 plate date as well. His frame, tank, transmission, and clutch are different from yours but he has stem mount handlebars like yours.
My 53 bike, which I was told by the previous owner was a USAF bike, is only a few frame and engine numbers from Randys. It is identical to RandyS' s except for the tail light.
I wonder if you have a newer mil contract engine with a older frame.
It would be nice if there was a list somewhere of all the different military contracts, dates, and numbers for Simplex.
Carl
Saint Simons Island, GA.