'56 was the first year of the 'Browning' import of the FN product. They were serial numbered from the start but a very very few have been found w/o any numbers.
Early rifles were #'d on the metal buttplate and in addition the front of the reciever face and sometimes the trigger assembly.
'A' prefix for .22short cal guns and a 'T' prefix for .22LR guns but again a few rifles exist with the opposite prefix of what is supposed to be correct.
In the late 50's and into 1960 the # was stamped onto the metal magazine ring plate at the rear of the stock in addition to the front of the reciver and trigger guard assembly. Some guns don't have the receiver'guard assembly # at all stamped into them. Some don't have the mag ring # but do have the receiver # stamped. Lots of variations.
The factory used to mark the ser# of the rifle onto the wood ( in chalk usually) underneath the butt plate during mfg'r for keeping track of matching parts. If the stock has been replaced or worked over, the chalk # can be missing easily enough.
In 1961 , the # '1' was added to the serial number in front of the letter prefix to denote the year of mfr. Also the .22short prefix was changed from 'A' to 'E'.
After 9T (1969) they went to 2 digit year prefix (1970),,70T, 71T, etc.
The 22short went to a 2 digit year prefix in 1969 with 69E and followed that pattern til 1974 IIRC.
Also,,GCA68 only added .22RF long guns and shotguns to the list of firearms required to be ser#'d (though many mfg'rs had done so anway).
Handguns and centerfire rifles had ser# requirements prior to GCA68.