Yours is the 110,234th Longbranch No.4 produced.
The woodwork will be Canadian Walnut, not Oak. Oak was never used.
If it closes on a no go gauge then don't fire it, get it checked by a gunsmith with proper headspace gauges who knows what they are doing.
Is it in proof? I have been sold two Longbranch Rifles in the last 12 mths and neither had any British Proof marks. This was remedied, and now both are proofed. If yours isn't then the seller has committed an offence and needs to get it proofed for you asap.
The woodwork will come off easily, at least the forends will. Give the wood a wipe down with something to get the gunk off such as white spirit with fine wire wool and then treat it properly with linseed oil, light coats and it will/should come up lovely.
The round cocking piece is unusual for a Mk1* of the age of yours, they changed to square ones before then so maybe it's been changed by a previous owner.
Some photos would be nice