Yes, zushi, not sushi. Saba means mackerel, so it's sort of like mackerel sushi, but different.
Sabazushi is made with saba that has been lightly pickled in salt, then filleted. The fillets are then pickled in sushi vinegar (rice vinegar and sugar) for a short time. Next the outer skin of the saba is carefully peeled away (retaining a delicate inner skin), remaining small bones are extracted with a tweezer-like device.
The fillet is placed into a kigata (a wooden form) for making oshizishi (pressed sushi) which is filled with sushi rice. A very thinly sliced piece of pickled kombu (kelp) is placed atop the saba. It is then pressed in the wooden form.
After pressing the sabazushi is placed in dried bamboo shoot covering and wrapped. The sabazushi will now keep (for a short time by modern standards).
Saba-zushi originally began as a way to get the “fresh” mackerel to the Kyoto-ites in their more landlocked position without having the fish spoil.