1948 M18 oil pump

If 'restored' means its cleaned up by shot blasting then take everything you were told about the bike with a huge pinch of salt. Dismantle the pump completely so you can close the clearances down to zero. When rebuilt properly the pump should be stiff to turn and will loosen off when its pumping oil.

There are lots of previous posts on here about oil pump refurbishment.
The pumps for Singles are different from Twins.  Singles used cast alloy, and twins used cast iron.  Probably because the shell bearings develop much higher oil pressure than in the ball and roller bearings in the Singles where pressure is only developed through oil flow restrictions.

Clearly this pump has not been refurbished recently. 
You can lap the outer plates to remove the radial grooves. You must do it on a perfectly flat surface, ideally a glass or granite plate. 
You ideally want a final lap to about 400 grit. 
The body and gears can also be carefully lapped for side clearance too. 
Before you bother though, inspect the gears for pitting. If they are not perfect find another pump. They are plentiful and better to start with the best one you can find.
Read previous posts for full description of the lapping process.