It wasn't just Triumph.
Cornbinders (International), Dodge, Plymouth, Willys, early Fords.
It's like the end of the month, see what we can do to fix failed blocks.
You haven't lived until you've taken apart an old Ford flathead V-8 and found sleeves on ONE bank, where "core shift" in the casting process made one bank too thin (or bored into the water jackets!).
Seen more than one of those.
The old books I have show various designators for changes in rods, mains, bore, wrist pins, etc.
It may have been a discoloration in the bores, caused a slight overbore to clean it up.
You never know.
They probably figured a certain percentage needed, oh, .005 or so over, ordered that many sets of pistons and rings, if it was more, the block probably got scrapped.