There are several numbers on the passenger's side of a 235-261 six.
This link shows a photo of a 1954 261 block, pointing out the location of the casting number, the distributor pad number, the date code, and what I have been told is a "conveyor line" code. Note that this casting number, 3835911, is usually listed as belonging to a 1953-55 235, but this block is definitely a 1954-55 first 261. I have heard of another 261 that alsp had this casting number.
Later model 261 blocks had the casting number further aft, just forward of the distributor, as Drew described.
Casting number location for a late 261. There have been several previous threads on the casting number questions. Search on "casting" or "number" or "date".
