Keep in mind that these charts are incomplete, and there is a reasonably wide margin of error between serial cutoff dates.
There isn't enough data to claim either option as absolute.
One may assume that "RA" serial numbers began with "RA00001" (or RA1000), and continue until "RA 99999 N," at which point the system was reset to "RB 00001 N" (or RB 10000 N).
This seems to have happened around 1962, and it is reasonable to assume that all "RB" symbols represent bikes produced post-1961.
This system showed its face in 1970 (I have named it accordingly), and was phased out in mid-1973.
If new information is found, these charts will be duly updated to reflect such.
This system follows a similar pattern to Convention #2 above, and uses a prefix or suffic of "RA" or "RB," followed by 4 or 5 digits, but never exceeding 5.
Location is on the side of the seat lug, as with the earlier serials above.
EXCEPTION: This system is NOT to be confused with the seat-tube system which replaced it in 1970; which ran until 1973.
Numbers At present, I have insufficent information about the serial numbers from this era to construct a definite chart of every example used during this time period. I have uncovered two of them to a reasonable extent; both of which ran concurrently to each other during this time period: The first system consists of 7 digits, beginning with 1000000.