It looks to me like the part you received clips the breaker bodies together (you would need to remove the breakers from the panel to do this) and the other part(s) are needed to connect the breaker handles.

No, I believe the piece in question is to handle tie two "skinnies" together that are not adjacent. The picture shows the three different handle ties:

The one on the left ties two adjacent skinnies together.
The one on the bottom ties two full size breakers together.
The one on the upper right ties two skinnies that separated by two other skinnies.

Ron is correct.

Thank you. I see now I should have bought the THT1. Not sure if I should just pick up one of those or start moving things around in the panel. It looks like the THT1 might just slide on without having to remove the panel cover?

I don't understand why you need to remove the cover. Even if you did, it's no big thing (I can't imagine what you'd be doing that involves tying the handles that doesn't also involve getting at the wiring.

I would have to move the breakers around to make this handle work. Right now they are next to each other. This is an existing multiwire circuit which I discovered when I thought power was off and noticed my furnace blower going on and off as I disconnected the neutrals. Not a big deal to remove the cover but I have a lot of things on my to do list already. I think I'll just order the THT1 and call it good. Thanks for the help.