Tabata Regs crossed years ago my way, when, for whatever reasons, one customer used the TR350/S30 Combo as fleet regulators and I had to service them.
I had no schematics or something else, but I was immediately impressed by the quality of their components. I don't know any other regs with a similar durable chrome coating as the ones from Tabata (TUSA).
The technical designs are interesting, the plastic used pretty strong and long living, but most parts are unique and not interchangeable with other brands or models (I think that's true at least for the Pre 2000 designs).
They had been very long living and on the test bench they always showed a good performance.
If I find Tabata Regs for cheap money in the bay, I still buy them, because I like to work on them.
I think Tabata Regs are quite underrated, they seem to have not a good promotion.
Maybe it's like with Sherwood, which is a huge company, and the Dive Equipment Department is just a very small part of it.
I didn't know that Tabata was producing for SP. I thought that Tabata and SP got their RS460-MK16 from the same Manufacturer in Taiwan.
Since the quality of the components of the RS460-MK16 1sts is at most industry standard, I doubt a bit that it was produced by the same manufacturer who produced the Pre 2000 Tabata designs, but maybe somebody here knows more.