This is the 220 Rook Rifle, or 22-4-30 ELEY Long. Listed in the 1899 Eley catalogue as a rook/rabbit rifle. According to Fleming, the earliest mention of this cartridge was in an 1884 Fraser advertisement. Believed to be an early centre fire version of the 22 LR rimfire cartridge, but was no longer listed in the 1908 Eley catalogue. Lot #75 of sale 5828 by Christies listed a 220 Rook Rifle. Unfortunately there was no picture to show what the gun looked like:

Lot Description

AN UNUSUAL .220 SINGLE-BARRELLED HAMMER ROOK-RIFLE BY WATSON BROS., NO. 2612
Sidelever action, non-selective ejector, best bold foliate-scroll engraving with some hardening-colour, the stock of unusual carbine configuration, with pistolgrip, horn-capped forend, the barrel with matt sight-flat and open sights
Weight 4lb. 11oz., 14½in. stock, 26in. barrel, Black Powder proof