Geoff;
Went home at lunch and took a look. If you mean a hole 4″ directly in back of the oil filter, 45 degree slant, there is most certainly supposed to be a bolt there!
Hex head, probably a 7/16 or 1/2.(?) The threads are probably deeper inside than you can feel with your finger.
Time for a trip to the auto store to get trial size bolts. Since the old one came out somewhere, be sure to thread lock the new one.
Good Luck
Ringo
The Haynes manual also shows this bolt (plug?) in place.
Ringo39948.7000347222