What is the purpose of the groove in the fore end stock on Ruger single shots ?

It's just asthetics, The rifle was desinged to look like the British Alex Henry single shot rifles.

Actually, the design is for holding the rifle secure in a gun rack in a vehicle when on safari.

I heard that the grove was to fit the gun sticks of the time. The Alex Henry fore stock has been a silly design to me from the start.

For me the Ruger No1 doesn't look right without the AH forend.

I could be tempted with a wide nice custom forend for a long range rifle however

Never been a fan of the 1B.

Cheers, Chris

Your Ruger fore end stock was circumsized. That's what makes it Kosher.

Another "Old Wives" tale is that slings occasionally were attached via a leather thong or lace which tied around the barrel and stock were thus held in position.

Leather and thongs... nice concept!