The "teeth" are properly called the hinge locking lugs on the stock extension assembly and the seat for hinge locking lugs in the lower hinge nut. But they do look like gear teeth, don't they?
Short version: careful disassembly, inspection, cleaning and reassembly can improve the working of a M1A1 folding stock.
Teal Deer version: I bought an M1A1 folding stock for my 1943 IBM M1 Carbine just to have an option (when I bought the carbine it came with a M2 "potbelly" stock).
I bought the folder at a gunshow, used but good condition, no brandname; I suspect it was an Italian replica and I paid about half the new retail price. The wood was nice looking.
I actually folded it only if needed (as when I carried the gun on an ATV) but eventually the hinge got loose. I decided to fix the stock in open position with a wood screw in the bottom of the stock to block folding. I used it several years as a fixed stock.
As part of this year's cleaning and preservation for winter storage, I followed US Army Tech Manual TM 9-1276 / USAF Tech Order TO 39A-5AD-2, Section XI. Disassembly, Inspection and Reassembly of Stock Group - Carbine M1A1.
Inspection. I made sure the recoil plate which the receiver hooks into at the rear of the stock was square with the hook on the rear of the receiver and retightened the grip screw. The grip screw ties the recoil plate, stock and pistol grip together. It is a wood screw so you have to be careful about over-tightening and stripping the wood of the grip.
I cleaned and inspected all metal parts. I found some wear but not much on the hinge locking lugs on the stock extension assembly and the seat for hinge locking lugs in the lower hinge nut. I scrubbed everything clean with an old toothbrush and Mobile 1 oil and wiped leaving a thin coat of oil.
I found that the sheet metal recoil plate cap sides were splayed. I pinched them so the cap was a snap fit over the recoil plate and the cap sides and rear were flush with the stock. The long hinge screw goes through the cap and screws into the lower hinge nut.
I reassembled everything and noticed the locking spring washer was laying on the carpet at my feet. Disassembly and reassembly require patience, properly fitting screwdrivers, and due attention to the instructions in the TM/TO.
Once I got everything back together the second time, paying attention to fit of parts and tightness of screws, the stock locked open without looseness or wobble, and locked closed without looseness or wobble. I did not reinstall my fixing screw.
