No ID stamping "could" mean it was a replacement engine. It was called to my attention that a 261 was a common swap in when 216s gave up. They were typically made with the 54 style water pump mount to be a more direct swap. I have a 54 style 261 that has as I recall the correct casting number but a later date code and no ID stamping as you mentioned. You may find something in this photo album that helps. It shows 2 different and different style 261s.
http://rides.webshots.com/album/578167968KpVMeA
