Blake's bounty hunter gun - almost there

Adding the strips of square rod to the rear of Blake's bounty hunter gun is where i slavishly follow what Bill Pearson did and move into conjecture as to just what was used.
First case in point, what's been called the "pixie hat" that sits in the blank area just above the trigger. Its called that for want of a better name as no-one has ever been able to figure out what was used - doorbell buttons, "Frustration" playing pieces have been put forward, but been discounted.
I've got some plumbing piping that was the correct 14mm diameter, so cut two pieces the correct 9mm height, filled them with P38, sanded the tops flat then glued both to washers to give the rim around the bottom.