Deve,
What is the Engine Serial Number on your 261?
Did your 261 come with a full-flow oil filter installed?
Also, what is the block date casting code (and the head date casting code, if you have the 261 head).
Some of us have seen or heard-of factory replacement 261s that had some odd characteristics (Greg Hill, if your read this, please comment on what WD found, if you remember). Maybe a replacement engine came with the plug out in case the engine got installed with no filter?
I am not surprised that an engine rebuild shop would not know about an oddity regarding a relatively uncommon engine made for 4 years. I'd bet they never saw a full-flow filter.
