As stated above there are effectively two types of fuel pumps used on 1850's and Sprints:
Short arm, original part number 312167
Long arm, original part number TKC3419 (which my parts book states was introduced around May 78). This type uses a spacer block (part number UKC8523) and correspondingly longer bolts to mount it.
Unfortunately most online descriptions make no differentiation between long or a short arm versions, additionally a lot of descriptions state the pumps are suitable for all years which they are, but what they neglect to state is if they need a spacer or not as the seller probably doesn't know they were used for some, but not all pumps (plus they possibly don't know which type they have).
All is not lost however as the spacer blocks are available to use the long arm version:
https://rimmerbros.com/Item--i-UKC8523
or alternatively you can just replace the shorter armed version like for like:
https://robsport.co.uk/index.php/online ... 051-312167