The “Spacer, sleeve”, (as per the 1973 “Illustrated Parts Catalogue”), is listed alongside the “Plate, seating" (NSN 1005-99-963-1592).

Thus a “proper" L-42 should have BOTH.

Why the extra “plate”?

The change of cartridge AND change of barrel profile meant that almost EVERYTHING related to No. 4 bedding went out the window.

The shorter fore end means that there is almost nowhere to play with barrel pressures, and, given the profile of an L-42 barrel, such pressures would probably need to be somewhat larger than for the trusty old No. 4.

I would imagine that a LOT of "carpentry" experiments were run, and the “failures” probably ended up heating the boiler to make tea.

The fore-end HAD to be stabilized and the combination of the “Spacer, sleeve” and “Plate, seating” appears to have done the trick.