The bolt is presumably a post-war replacement from the shape of the knob. The funny thing is, the finish on the bolt doesn't match that on the action. Are the numbers on the bolt handle struck through the finish or under it?

The staked pad screws would also indicate post-war refurbishment, which in turn would suggest a rifle that saw a good deal of service in WWII.

Interesting that it has the "Mk.I" fluted trigger. Some snipers were said to request those be fitted to their rifles.

The differing size of the "S" in the "S51" might indicate an armourer making things "tidy" after a replacement butt was fitted, or it might just be a variation in H&H's practice. Someone else might know?