I think maybe they are just some grind marks Pat, I don't see a triangle there. I do see that the '900' portion of the S/N appears to have been stamped over top of something else that didn't get fully scrubbed. The second zero sure looks like it's over top of a zero itself (or possible the top of a 2, 3, 6, or 9).
The pinned barrel with the earliest S/N I've found is on the 1970's stamped guns out of [0138]. By year 16 (1971), they are quite a bit more numerous at other arsenals.
It could be that this one was an earlier pinned variety that was scrubbed and completely overhauled in year 19 like LC says. IT could also have been a year 19 carbine and during refurbishment the original S/N had to be scrubbed and restamped because of a very poor condition receiver. Seems like a pretty plausible scenario.
Nice looking carbine fyrwokr, welcome aboard.