63 was first year 300 was a model without the letter, letters where on the "real or top of the line" 300 through 65. 1966 and on 300 was just a model
Close, but it was '62 that started the standard 300 series and the H, J, K and L letter cars were the limited production high performance series continued on from the '55 C-300, '56 B, '57 C, '58 D, '59 E, '60 F, and the '61 G.
The H was the '62 performance letter car 413, and without the H it would have been a standard 300 series with a 383 as standard equipment and was classed as a separate model like the regular '70 300 vs. the '70 Hurst. They kept this option through '65 with the 300L being the last letter car. Production figures should show them as a separate model. In the standard series I would expect that a 413 was an option, along with buckets, console etc, but it wouldn't make it a letter car.
For some reason I think the cross-ram that started in the '60 F, skipped the '62 H and was an option again in the '63 J and the '64 K, but back to only the 4bbl in the final year. Not positive, but fairly certain that I read that from a legit source at some point many moons ago.
