I got lucky and found a 261 to replace my tired 235. The casting number is 3769925 which is listed as 58-62, but the Date code is F 9 7 which
I think decodes to June 9, 1957. It has the large oil line receptacles and is a full pressure filter engine. It has never been bored and came out of a 1962 C60.

So I think it is vintage 1962.
What do you think?