Denny,

The 1957 edition of the Chevrolet Master Parts Catalog (thanks again, Keith Hardy and TOCMP) shows the "848" head as the replacement head available in 1957 for all high pressure 235s (1953 Powerglide (and Corvette) 235s and ALL other 235s through 1957). So, the 848 was obviously available in 1957.

The big horsepower increase in all 1956 235s was probably at least partially a result of the introduction of the higher compression (848) 235 head?

Tim