Lots of ways to get there and a Benelli Semi in a 20 would be hard to beat, but they don't say Grouse to me. Here are three that do.
The new one: Ithaca Model 37 in a 28 gauge:
One that is old, but one of the finest. A Model 12 in 16 Gauge:
If you want to go evil on Grouse, this one would be hard to beat. Winchester Model 59 in 12 Gauge:
And a joy to hunt with. Have lost count a long time ago on the Grouse it has taken.
Parker Reproduction in 16 Gauge:
Find one that fits close and then shoot it a lot. All three of these are different when it comes to dimensions. Grouse for the most part are about hearing, looking hard at the Bird and instinctive shooting. Rarely do you get the chance to have a perfect mount with feet and hands perfectly set as on the skeet field.