I've finished the Reilly history. But this gun on Gunsinternational is interesting:https://www.gunsinternational.com/g...ar-drop-locks.cfm?gun_id=102028688#sm-19
The SN 20659 dates it to early spring 1877. The weight is listed at 7 lbs 8.75 oz...which is far heavier than a standard Reilly field gun. It is chambered for 2 3/4" but there is no indication it has been reproofed so that is likely original. The chambering needs to be confirmed from the seller. This is of course black powder. Chokes are "1/4" and cylinder but in a 1877 gun without a "not for ball" stamp, surely that "1/4" has been caused by honing the 30" Damascus barrels. The stock may be a replacement but well done. The locks are best quality Stanton's (Reilly would have charged a premium for them) and polished internally.
I assess this as a 1870's pigeon gun because of its weight and chambering. Reilly began advertising pigeon guns in 1868 and this became one of his main sporting advertisement promotions. Looks like a lot of drop.
The seller has his Reilly history mixed up - I sent him a correction but no matter...read the Reilly history line. The price at $1,500 appears to be is quite reasonable.