The Rochester model B carbs. used several jet sizes.For a carb. used on a 216,which had a smaller throttle bore size than a 235, the #51 was the standard size - the #50 was a one step lean.The number represents the size of the opening in the jet in thousands of and inch.I am sure either one could be used at seal level or 5000 feet with out affecting performance.My experiance in driving at high altitude with the older cars has been that the most noticable difference is above 9 or 10,000 feet...the exhaust will start emmiting black smoke due to the lack of oxygen-which causes a rich mixture.For the best performance at 5000 feet advancing the timing (with the octane selector) an extra 5 degrees will do more for the performance than a .001" change in jet size.Due to the lack of oxygen/richer mixture more advance can be used.
The 235 Rochester used anything from a #56 to #60.
Some makes of cars back in the '30's offered high compression cylinder heads to make up for the power loss in high altitude driving.
