You are correct, you need to purchase and inner sleeve. Both the 1 1/4 and the 1 3/8 will work with the outer sleeve. I have been using them for about 2 years now, and they are great for people that have alot of equipment to maintain the feel ball to ball.
Also, you can buy an inner sleeve without a slug mounted in it, with this inner sleeve you can use custom thumb molds to make a custom thumb mold interchangable sleeve, this is what I use.
Best of both world's in one nice interchangable thumb.
Hope this helps you out....
