500,
What your seeing is a 3/8th dovetail as opposed to a catapiller cut, and that is very unusual on a double rifle..

I have been using Wipe Out since its conception, great stuff...BTW Forrester foam is not as good..Carburator fluid was pretty good stuff in its day and still not bad, but Wipe Out is in a class of its own..

Now a homemade solution of One quart of 25% Amonia and a cup of Hydrogen Peroxide with a Penny tossed in will do the same thing as Wipe Out, maybe a bit better..Its been around for decades and used by Benchresters who named it Blue Goop, but don't leave it in your bore more than 15 minutes and don't take a deep snort of it..