If he has a B-Square one that Brownells sold before the present one, the only choice is to try an industrial supply co. Brownells is no help on them, the current ones parts won't work. I have a 12ga one and had to buy several packages before I found the right one. If it will help I will check the ones I have and give a ball park idea of the size.

I learn something every day, and a lot of times it's that what I learned the day before was wrong