Hi Roy

Excellent rifle in superb condition
Is it converted from the Naval rifle? or is it a possibility that it was bought in as a sporter, although such guns/rifles were usually issued to Armed Merchant Ships?

The No. 4 SC is not the rifle number/type but possibly a unit/ship number??

If it is military issue it is a SMLE Mark 1*** (introduced in 1914) which differs from the SMLE Mark 1 in not having the charger loading bridge introduced in other SMLE variants in 1907
This variant was only issued I believe to the Royal Navy
The magazine cut off was kept until January 1916 when it was removed as a wartime expedient along with the long range dial sights and windgage

The Lee Enfield No 4 rifle (not a SMLE) was finally introduced after 15 years messing about in 1939

You have a unique and rare rifle, good luck shooting it

Tony