Number matching engine in my possession

[Road Reptile]

Hi 289 "k" fans
Just a simple response to your question about the letters cast and stamped in the block.
Have no ideas on the O cast before the C5AE-6015E But I found  SIMILAR sample in Bob Mannel's book in chapter 5 page 5-9  so it occured at times during production. All stamped numbers or letters are from dates of various inspections or tests done during final assembly. The final assembly date is stamped into the block on the pad of the deck
And most times carries the initial of an inspector following the date. The block condition is still the most important factor, and hopefully you get good news from the shop after cleaning/inspecting are done.
R.R.