AFAIK, we do not have clarity on why short stocks that were apparently newly made were installed as replacements (ca. 1901) on certain early carbines, after that stock was technically obsolete.

Use of the sling was abandoned with the M1899. My guess is that they discovered some partly-machined 30" stocks and decided to use them, sans bar.

The problem with stock length swapping is not handguard driven - rather it is in the grasping grooves, which alone determine period of stock manufacture - there are no long stocks with short grooves, and a 30" stock with long grooves would stand out like a sore thumb.

Bottom line: the stock in question is real, but who applied it, where, and why, is not clear at this point.