I mined this out of CR list archives:
The Dancing Chain says on page 179:
"1963. The significant difference between the Gran Sport and
the Record was the location of the cage pivot. The cage was revised
to move the pivot farther forward and closer to the jockey pulley so
that front shifts had less effect on chain gap. The Record had a bit
more capacity. It could handle a 28-tooth rear sprocket but shifting
degraded of the chainwheel difference was more than about six-teeth"
Some of the posts there suggest that the Record can handle up to 34 teeth with
no problem as long as the chainring difference is 5 or 6 teeth or less which easily
fits your 49-51.
Marty