hate to keep on asking questions, but i am confused. checking the engine serial number located on right side of engine above the dipstick - reads 0180511F55X. F for flint plant, 55 year and the last digit should be engine size. the catalogs i have, there is no x listing. 235 should be z. looking at my 54 chevrolet truck manual, they show a serial number for a 54 also ending in x, yet no listing for prefix x. what does the x stand for??? thanks in advance.
