I have never heard this style termed a Griffin Poacher's gun (I believe some of those did have skeleton stocks though) but have seen at least 4 guns by Griffin with larger bores that firearms experts have called coaching carbines.
A 15 BORE FLINTLOCK COACHING CARBINE BY BENJAMIN GRIFFIN, BOND STREET, LONDON, CIRCA 1765 with lightly swamped barrel fitted with silver barleycorn fore-sight, signed within a linear frame drawn-out to a loop over the breech, fitted with a moulded cup beneath the muzzle and a moulded ramrod-pipe behind, engraved grooved tang, signed border-engraved rounded lock fitted with moulded sliding safety-catch and later cock, figured walnut half-stock, skeleton butt, engraved brass mounts comprising fore-end cap, trigger-guard with a rose on the bow and early acorn finial, solid side-plate, and later horn-tipped wooden ramrod 62cm; 24 3/8in barrel Provenance William Keith Neil, sold Christie's 8th November 1995, lot 123 William Wrightson, Cusworth Hall, Doncaster Literature W. Keith Neal & D. H. L. Back, Griffin & Tow and W. Bailles, 1989, p.70, plates 20a, b. £2500-3000
Sold at Thomas Delmar