The holes are there so it is the 'extension' chainguard .
According to Norton Service Release N3/55, May '73, the 064837 chainguard with holes for the extension was introduced "
subsequent to engine number 307091" so pre-Mk2/2A.
As you said, the extension parts are missing from the Mk2/2A supplement although the 064837 chainguard is listed.
The later 065818 slotted chainguard also has the holes and is for the 850 Mk2 and Mk2A (from January '74) not just "850Mk2a"
Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
064837 therefore, is also not just for the "850 Mk3".
Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
The Mk3 reverted to the earlier 064837 non-slotted chainguard as with the rear disc brake the slot was no longer required.