My findings were that there is a rubber skirtish looking seal which has deteriorated. It is the seal that seals the pump shaft where the pump mates with the head. There doesnt seem to be any scoring on the cam surface, and the radial pumping action works when I spin the pump shaft. The pistons are nice and flat where the cam makes contact. I can probably service the pump in its condition.

If anyone is interested, here are the Mercedes SLS pump gasket part numbers for an 82 300TD:

SLS gaskets

(1) 004 997 01 47 seal ring
(1) 010 997 43 45 seal ring
(1) 114 236 00 80 gasket