'pappypete' - You have given us all 'a puzzle to ponder'.

My theory is:

In 1895 a distracted Springfield Armory workman failed to machine the newly introduced 'notch' on receiver #24432 and it went on to be heat treated.

It was assembled into an otherwise respectable early model 1896 carbine with a model 1896 bolt and rear sight. The pin on the 1896 extractor caused a bit of wear and brightening where it rubbed on the top of the 'un-notched' receiver.

In a later era, a 'collector' or Dealer, (noticing the carbine's receiver lacked the hold-open notch), attempted to make it "more Arsenal Correct". He put the 1892 bolt and rear-sight on the carbine, thinking these parts went with an un-notched receiver. He 'created a carbine that never was' - IMHO.

(This is much like some Present Day M1 carbine collectors do in rearranging parts to make 'correct guns').

Today, we ponder, scratch our heads, and wonder?????