If the number is correct she's based on an RAF 60 foot General Service Pinnace.
But the hull shape doesn't fit! See attached pics of number 1261, closest I could find to 1258 but from the same batch.
Nearest I could find that fits the hull and superstructure
shape of the model
at the bow is the 65ft Walton HSL (High Speed Launch).
See pics attached pics of #2642. She was a 'One Off' which never went into production.
Hope this helps, or does it just add to the confusion!?🤔
Cheers, Doug 😎