Could be a re-stamp???
Check out the stamp (on the left) If you look closely at the photo it appears as if certain numbers are super-imposed on top of other numbers. As if it were re-stamped at some point. I think that a Q stamp on a 656 block is most likely a restamp that someone did because they wanted a legitimate looking "Q" engine stamp for a '64 or earlier car. It would not have come from the factory stamped that way.
The date code on the 656 block shown is 253 which is Sept 9, 1964. This would be consistent with a 124xxx serial number car that was assembled in the Janesville, WI. assembly plant and spent it's whole life in Indiana. But I find the overstamp kind of odd. 656 Blocks with a casting date of 2 to 43 would be a very late 1965 block, as opposed to a very early block, as conventional wisdom would hold.
Reason??? Friday, Feb 12, 1965 (Julian date 43) was the very last day that 409s were cast at Tonawanda before moving the "W" line off the main production floor there. 656 blocks, most likely, would have started production on July 19, 1964 (Julian date 232) Im am not 100% sure of when the actual day they started was , Fran Preve could tell us that. I am positive, however, about the end date. So, 656 blocks have a date range of 232? -366 and 2-43 Mine is a 25 which is Monday Jan 25, 1965 my block stamp and protecto plate is T0205JE Right on the money for my car.
The only factory truck stamp that I have ever seen on any 656 notched block was XXG, that doesn't mean that there aren't other truck possibilities, those are just all the ones that I have seen.
Rock