That's correct they're not interchangeable, sadly.
I think sometimes ppl buy them before they realize that. The Z3200 I rescued also came with an HP70 MK-R in the box (an after-market one actually).
HP 70 MK-R heads (and R ink) are the only thing keeping Z3100s alive today (without going after-market). They're crazy scarce so sell quickly.
If sealed I'd buy them, but I already mothballed my Z3100 a month ago after the formatter drive dried (and that was after the carriage PCA was replaced and 5 fried heads). I'm also having ink delivery issues with my last 3 MK-R heads, so figure it's just a money pit at this point. (I have a theory that it needs the carriage ink port valves replaced since they are leaking, and the heads are probably fine...) Anyhow I'm putting everything into the Z3200 now for the little bit of printing I do. It was always intended to replace my Z3100 anyhow.