During WWII, the methods of production and assembly were 1930's level technology. Winchester parts on SA guns does not seem to have happened much.

Imagining what went on at SA is not hard.

Assembly area "bins" at SA were likely kept stocked with parts from a central parts collecting area and distributed by entry level, "parts boys".
The constant "rush basis" was worst at the beginning of a shift. End of shift is always the same everywhere and the next shift got to bust their *** to catch up for their shift, and "out of order" parts delivery happended naturally.

As the boys caught up, they went back to an existing inventory distribution basis, always keeping a backup supply of each part for "just in case we run short", days.

Also , repair parts had to be produced for field or depot level repairs and that mixed things up real well, as parts produced went to packaging and went out the door, never to the assembly area.

That central inventory pile that went to the assembly parts bins was replenished on an as needed basis and could have parts months old, or new.

Receivers being produced were kept on boxes on dollies and last in first out each day was likely the procedure. The barrel date rule of +1 to -3 months around receiver date seems to have been generally ok, with some receivers stuck in a corner, and extra barrels stored too until the chance of rusting came up, and a rack of whatever was sent to assembly.

One to three months date variance on barrels/receivers tells the whole tale! Madhouse!!

Receivers and barrels were kept as a backup reserve and changing the reserve each day was likley not done so much as augmentation and rotation by supervisors, to prevent rusting.
A real Garandicon in 1943 could be a real mixmaster of parts produced within the 4 months and more likely 2-3 months on either side of the receiver production date, at the mean. Statistics rules apply here.
Guns that did not meet inspection and sent back into internal repair or rebuild as new also affects what came out to finally get a Cartouche. Later cartouches on earlier guns would be expected on a small percentage.

I suspect also that every third or fourth gun off the line was sent to storage for domestic emergency use. That would be about a 750,000 new guns that went to a strategic inventory.
That would explain the new condition I have seen on demilled receivers from 1943-44, including winchesters, and the new 1943 barrels that keep showing up.
I'll bet that this inventory came from the Carter and Clinton era destruction programs.

There are really very few original guns left and the temptation to improve and restore guns has been strong.

With prices dropping on everything garand, we may end up with Krag values as the AR15 crowd bypasses the garand altogether.

But there is no worse investment than a new $45,000 truck.