If there's no chamber length marked, then it's a pre-1925 gun--which agrees with the SN you posted--and I'd be very skeptical that it had factory 2 3/4" chambers. If they've been lengthened, the gun would be out of proof under British rules, although it might very well be safe to shoot (but NOT with American factory 2 3/4" ammo) depending on overall condition, with particular attention to barrel wall thickness.
I believe the 400/500 guns came along after WWI--someone with the Scott book may be able to confirm that.
$1,000 would be a good price for a Scott nonejector gun in decent condition. But if for nothing else but the chamber length issue, I'd want to have a knowledgeable gunsmith take a look at it.
