What are the dimensions of the aliminium subcal sleeve…firstly, it looks like it is turned from 10mm (or 3/8" (9,52mm) extruded Aluminium rod. so could be made on a “screw machine” or a Turret lathe, or even a simple "Hobby lathe.
The rebated bore of this subcal looks very much like a #209 or #245 Shotgun Primer battery cup ( about .300 rim and .243 cup diameter).
It might be for some sort of .22 ball or .25 ball subcal charge for Indoor practice…a Modern form of “Conus” design.
I myself have made (using recycled .177 air-rifle barrels, subcal devices in .30/06 and 7,9mm to make short, rifled “Faux cartouche” using a shotgun primer and a .177 pellet, loaded from the rear, and then fed from the magazine into a Short Rifle ( say a Kar98 original 1900 model, or a Columbian FN Gendarmerie carbine, also very short barrel.) For “Zimmer-shutzen” it is ideal; the Primer has enough energy to discharge the .177 pellet down about 2,5 inches of barrel to gain the rifling, and it travels quite accurately up to ten metres ( I use it at about seven to 10 paces, about 8 metres in a Workshop shed). Keeps your aim in.
I have a German 9mm K subcal device, fitted to take a RWS 4mm M20 CB type load; the “Faux cartouche” is turned steel. it cycles just like a real cases (by hand); it was fitted to a Walther PPK set.
The steel device looks identical to the aluminium one shown.