Chevy block stamp

[al8apex]

this engine is in my neighbor's 65 Corvette convertible and the vin sequence doesn't match the vin under the dash, thus is isn't the correct engine

that said the stamp reads 18R131759 to808hc

HC equates to a 327 - 275 hp powerglide (what the car actually is, a low horse PG), assuming 68

what was this engine originally in? (the 18R part ...), ok R = Arlington, so this was a B body Chevy? (Impala / Caprice)

thanks in advance