1375548 - 63 425 1375548 - 64-66 401 4bbl I found this here: http://buicks.net/shop/reference/nailhead_casting_numbers.html
The first b is probably a 6, thats the way they looked plus they did not use lower case letters. H is Flint, the rest it the VIN for the car it came from. On the same surface as that stamp should be some letters like M R or M T - find that and report back here for more info! M designated 1966. MR 401 from a GS MT 401 from full size car MW 425 single carb MX (rare!) 2-4bbl carb Other letters intead of M: J '63 K '64 L '65 M '66
