...that's not something I've encountered before. Every DT braze on from that period I've seen, those things come off, and are fitted onto a square braze on like the ones shown above.
I hate to ask this, but I will anyway. Is it possible whoever painted or powdercoated it neglected to remove them, and just painted right over them ? If that turns out to be the case, you'll have to carefully cut the paint in the back of them and carefully pry them off. The rear derailleur usually controlled by the shifter that fits on there was very often something like a Cyclo Benelux pull chain/push rod affair.

I'm at a loss to figure out why your frame has cabling stops on both sides. Usually in the early 50's (on up through about 1957 or 58), what I've seen is some kind of lever mounted like that to operate the rear, and a manual rod you flip for the front changer (if so equipped). But things were in flux then, and certainly the Campagnolo Gran Sport parallelogram design came on the scene early in the 50's. The bicycles from the 50's I've had and worked on were slightly more mundane, production cycles from Raleigh (Lenton)and Follis.




^^^^ This was how Simplex was doing it up through about 1959.






^^^ this was the Lenton setup in 1958. I think they went to dual cable operated derailleurs the following year . But they were around before that.
You can probably help yourself a lot by doing some image searches for that period. There are a lot of period images available on the internet.





...seatpost sizes are all over the place in the 50's. I just had to buy another (smaller) adjustable reamer to work on the seat tube of a Follis.

...FWIW, you can go over to Disraeli Gears and cruise through his derailleur listings by decade. 1950's link.