I believe the marking indicates

...46.5 grains of NC (Nitro Cellulose powder (smokeless powder)
The Crown/BNP also indicated Birmingham Smokeless Powder Proofed.

...245 grain Bullet
The " sign is used under the somewhat poorly stamped GRs right above it to save space I imagine and was just an easy way to mark.

If this rifle had gone thru proof in the early 50's, there would have most likely been an additional marking of pressure.
It would be in English Tons (Tonnes (sp?) and then followed by /per in sq or sometimes the 'sq' is a simple small square block stamp mark. Sometimes the inch stamping is that "/quote mark again.
1 Engl Ton/sq inch = 2240psi

Why the marking is not present I can not say. Perhaps it was dropped at some point in the 60's or 70's. I usually deal with guns proofed much earlier and those Military Surplus proofed right after WW2 and sold on the commercial market. They all seem to have the marking.

Another point is that since the M/S is a foreign made firearm (Not English Mfg), the proof marking themselves would be inside bold circles IF England did not recognize the proof methods and specs that the firearm had undergone when mfg'd.
Birmingham obviously did recognize the Austrian proof of the carbine and the Birmingham proof marks are NOT within circles.
Any US mfg's firearms sold in England, and US Surplus Military arms sold out of service in England to the civilian marked had to be proofed before that sale. Since the USA has no Govt mandated proof, England will not allow any US mfg firearm to be sold commercially (new or used) with out first undergoing UK proof.

These will all show the proof marks inside the Bold Circles you've probably seen on some firearms.

The Birmingham Proof House used a date code symbol starting in 1921.
It changed a bit a few times over the years but is still in use AFAIK.
You can find a chart of it in a google search most likely. I have one in my notes for reference.

London Proof House never used a date code as far as I know

Changes in Proof Law which forced changes in individual marking stamps or adding & deleting some can be used to pin down general era of proofing for either House.