It could be a personal prize from a Regimental or perhaps Army or Inter Service Shooting Championships.
The obvious contact is the Guards Museum to see if they can shed any light on it, but I suggest you write to them and send pictures of both sides of the spoon and close-ups of any detail. If you know his service number and rank mention that in the letter.
Although service records of men who served after 1923 are with the MoD, the Coldstream Guards HQ may have some information or even a duplicate service record for him, write to:
RHQ Grenadier/Coldstream/Scots/Irish/Welsh (select as appropriate) Guards
London SW1E 6HQ