Hi Peter and welcome to the forum and an interesting question!
I have not seen valve discs modified like this before and can only guess at the possible reasoning of the person who modified it. Quite often these discs are missing and I have only come across one that has been modified and that was in the valve at the bottom of the hand oil pump in the tank. This has had a small hole punched in it (see picture) I have no idea why this was done and would most likely cause some oil to be draw back into the pump from the oil sight when the pump is lifted to refill from the oil tank, rather than heading on a one way trip to the engine as it should - but there may be a perfectly logical reason for it?
One thing did come to mind when I dug the bits out for the picture, is that the discs in the non-return valve are smaller diameter that the ones in the oil pump. I don't know the diameter of your disc, but if it was a spare one out of an oil pump, maybe it was modified to suit the one way valve? A simple measurement could give a clue or blow my theory out of the water! :-).
There is a screw-in brass disc seat in the female threaded half of the one way valve (didn't come out well in my picture and it wouldn't unscrew easily - a common problem with 100 year old brass parts). If that is missing and the bigger disc from an oil pump is used, then that might explain things, as the little "V's" in your disc, would help let the oil get past when the disc is off its seat (on the male threaded part). This job is normally performed by two slots in the screw in brass seat insert and the bigger disc would not fit in the recess.
As long as the little "V's" don't encroach on the hole through the male threaded part then it may actually seal ok???
If the disc is 3/8" diameter then my only other guess on the purpose of the little "V's" is to allow for extra oil flow through the valve when trying to refill the sump after it has been drained with the obligatory 3 full pumps of the oil pump and the ambient temperature is low. As mentioned above if the "V's" are not deep enough to allow oil through to the hole in the male threaded part of the valve when it should be sealing, it may well work ok (??....and maybe not!)
I would be interested to see what others think (...and happy for my theories to be debunked! :-) ).
Edit:- I just had a closer look at your drawing and possible the spring is not on the correct side of the disc?? Cross section of valve here;
https://www.douglasmotorcycles.net/index.php?topic=2140.msg7758#msg7758What Doug refers to as the "cage" is the screw in brass seat I referred to above.
cheers
Hutch