Hello Joerg,
Those FR3 engines sound great! As I understand the numbering system from that time, the first three digits are the serial number in that year's production, then the letters are the model, next digit is number of cylinders, next letter(s) are for options or application, and last digits are added to 1950 to give year of build.
Thus you have-- Serial numbers, 381 and 382
Model, FR,
Cylinders, 3,
Type, ME, (Marine, not sure about the "E")
Year 10 + 1950 = 1960.
From the numbers they are a pair of twins. Do they both rotate the same way?
The British "Internal Fire Engine Museum" list FR manuals available. You will need to pay a subscription fee to get them, but they are very good.
The 3 cylinder motors should run very smooth in a boat, and sound wonderful! Regards, Combustor.