That plane is affectionately referred to as the LTU. I believe there are 4 of them and that were bought from the German carrier that went out of business several years ago. The seats are old and ever so thick, so that as you stated, there is like zero leg room.

I tried to see if there is a way to tell what flights will be using the LTU's, but I don't believe that there is.

Strangely, the LTU's first class section has more room than either of Hawaiian's other types of 6-7's.