Castle SO SCOTCH WHISKY

The landlord of my local pub ,The Star Inn, in Rainford, Merseyside, was recently gifted a bottle of Castle SO SCOTCH WHISKY from a house clearance. The bottle has a glass stopper and appears to be a good age. Having opened it and dispensed it to the regulars for tasting it was found to be an extremely palatable scotch with a peaty flavour though not full strength.
We are all a little intrigued as to it's origins but internet searches have produced little information. If anyone can enlighten us we would be grateful.

The bottle label reads as follows

Castle SO SCOTCH WHISKY
QUALITY Fine old malt Whisky
STRENGTH 18 under proof by distillation when tested
by Sykes's Hydrometer for bottling
MEASURE Six bottles contain approximately one gallon

Bottled and Guaranteed by W & A Gilbey