When I first saw the weight and the semi-pistol grip I wondered if it were originally a rifle. However, the proofs on the barrels show it was indeed pre 1887. I may have been meant for the pigeon ring although it doesn't have some known Reilly Pigeon gun features but it certainly is possible that the owner wanted to use it for ball as you speculate.
In June 1875 "Not for Ball" stamp was added to choked guns. Greener had come out with his choke boring system in January 1875 and in a series of competitions proved their efficacy. Unlike other innovations all of the UK gun industry jumped on it and began advertising choked guns. Reilly's first such advertisement was in May 1875. This was done away with in 1887 replaced by "Choke." However, if a gun is cylinder bore, there will be no "Not for Ball" stamp.
I'll post two contemporary Reilly Pigeon guns below for comparison:
SN
23355 -1881: Note the weight, chokes, flat top rib and chambering (reproofed).
https://www.bonhams.com/auctions/24227/lot/462/?category=listA 12-bore hammer pigeon gun by E.M. Reilly & Co., no. 23355
Toplever, rebounding backlocks, percussion-fences, best foliate scroll-engraving, traces of hardening-colour, the well-figured stock with chequered ebonite butt-plate, the damascus barrels with game-rib engraved E.M. Reilly & Co., Oxford Street, London & Rue Scribe, Paris
Weight 8½lb., 14⅜in. pull (14⅛in. stock), 32in. barrels, approx. ⅝ & ¾ choke, 3in. chambers, London nitro reproof
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24650 - 1882: Typical of post 1881 Reilly top quality pigeon guns - Whitworth Barrels, fences, 7 lbs 8 oz (Hurlingham weight) etc:
https://auctions.holtsauctioneers.c...221+++1810+&refno=171703&image=4E.M. REILLY & CO.
A 12-BORE SIDELEVER HAMMER PIGEON GUN, serial no. 24650,
circa 1885, 30in. nitro reproved barrels (some light pitting, minor ring bulge in left barrel) with raised tapered file cut matt rib, the tubes engraved 'E.M. REILLY & CO. OXFORD STREET. LONDON. & RUE SCRIBE. PARIS. COMPRESSED STEEL.', 2 3/4in. chambers, bored approx. 3/4 choke in both, broad treble-grip action with oblique dolls-head third bite, side clips, carved percussion fences, rebounding sidelocks, broad strikers, border and fine scroll engraving, retaining slight traces of renewed finish, 14 3/8in. highly figured stock including 1in. rubber recoil pad, weight 7lb. 7oz.