The WR the bolt locks into the action as it does on all Mausers. The collar with the lugs and recess is just for the lock-up of the barrel.

All takedown arms with screw threads have the problem of thread wear. In the lugged collar types the female collar and the male collar with the lugs were hardened to minimize the wear.

I do a take down Mauser a bit differently.

I use 1/4 inch steel plate and surface grind 2 faces so they are dead flat. One is set and screwed with an epoxy bed to the rear section of the stock in front of the action very much like the Rigby shown there. On the other, I weld an "arm" to inlet into the forend wood at the 6:00 position under the barrel and use screws and epoxy on it too, so it's set flat to flat against the one in the rear of the rifle. The 2 steel pieces are carefully fitted to match the barrels (I do 2-3 barrels in different calibers as a rule)
Next I drill a long hole down the length of the forend and use a hollow piece of steel tuning cut with a tapered 3/8" thread. (Think of a ram-rod in a muzzleloader to get an idea.) That piece of steel goes through the forward piece of steel and is then threaded into the rear piece and the tapered thread can be tightened as it wears so it never gets loose with age. There is a tapered "head" just where the tube impinges on the steel plate so it self-centers. The steel rod pokes out the front of the forend under the barrel, and I set a sling swivel stud into it on a heads that is also adjustable, to keep the swivel in the 3:00 to 9:00 position. So if the owner gets some thread wear in a few years he/she can adjust the swivel stud to it's horizontal position to compensate. The tapered thread on the rod pulls the set-up into a perfect 6:00 position every time and keeps the barrel assembly from getting sloppy over time. To remove barrel you need only remove the sling from the front swivel, unscrew the rod about 4 turns which unlocks the 2 steel plates. Then you just unscrew the barrel and forend assembly from the action and replace it with the other barrel. They stay zeroed with their respective scopes and irons every time.

In doing the take-down that way I can make a rifle with 2 or 3 barrels, so the owner can swap from (as examples) a 6MM Remington, a 7X57 and a 9.3X57. Or 25-06, 30-06 and 9.3X62. Or even a 7MM Rem Mag, 338 Win mag and 458 Win Mag.

I am making one right now in fact.

It's only a 2 barreled rifle with a 257 Roberts and a 9X57 Mauser. The 9MM I intend to set up with a scout scope and both barrels are set up with standard front sights. The action is made with standard scope bases and I make a peep sight to fold down on the rear base to clear the scope. So the 257 is able to use a 2X-7X scope on the regular base and the sight are a back up system. The 9X57 has the scope base set, screwed and soldered to the barrel and has detachable rings to drop that scope off if need be. All zeroing of the iron sight is done with the front sights on both barrels, and the peep sight works with both. So the regular scope comes to zero with the 257 Roberts and the scout scope is zeroed on it's 9X57 barrel. The irons for each caliber are zeroed for windage and elevation with the same rear peep sight.

Such a rifle is a handy tool for hunting game from coyotes to moose. As a rule I work up a good load with every barrel and provide a set of dies for each caliber and the load data with the rifle.

My idea for this set is the 257 zeroed for 120 grain Nosler Partitions at 200 yards with both scope and irons, and the 9X57 zeroed at 100 with it's scope and irons. The 9X57 is made with a 35 caliber barrel with .358" groove to groove sizing, to use the more standard .358" bullets not the older .356". I intend to set it up the 9X57 for a 100 yard zero with a Nosler 225 Grain Partitions.