Mr google found me the bugari and pigini ones, found zero sette a bit by accident. I couldn't obviously find one on the hohner site (and they haven't replied to me asking if they still manufactured them). The zero sette one brand new is a lot cheaper then the bugari but apparently they share the same factory.
Special instruments are not always easily find just by looking at an online catalog of a company. Just out of curiosity I looked at the Pigini website which lists the P34 bass accordion with a range of 39 notes (C1 to D4) whereas that used to be an instrument with 34 notes (hence the 34 in the name) and it lists the C39 but not the P39 which of course also exists...
The Bugari website lists both the PA and CBA version of their bass accordion.
The Zero Sette website lists a bass accordion with a range of B0 to C4 and doesn't say that the B0 is likely a dummy (I don't know whether it is or not, but I know of no reed maker that makes a B0 reed). If it is a lot cheaper than the Bugari I suspect it may not have the "Umlenkstimmstock" construction, but again, I don't know. In an case, the bass not going up to D4 is a problem with modern compositions and arrangements.
It is quite likely that Hohner no longer sells a bass accordion, I don't know. They used to build a smallish bass accordion with 34 notes that many orchestras used (it only goes up to A3) and that had a nice deep sound. Then they replaced that model by a bass accordion with 37 notes (C1 to C4) and I don't know who manufactured that. It was popular in German accordion orchestras but I cannot remember ever seeing it much elsewhere.
In terms of popularity (I have seen many accordion orchestras) the Bugari bass accordion is by far the most popular instruments. Only in recent years the Pigini P39 and C39 started to appear. I think they only started making these about 15 years ago, to have a bass accordion with 3 reed sets and with registers. I have one of the relatively early ones, with reed blocks that are glued in (aaaarrrrrgggghhhh!!!) and with regular and chin switches. I believe the newer ones are no longer glued (have not seen that) and only have chin switches. I don't know whether they solved the tuning problem with the M and H reeds.
(The only other bass accordion with registers I ever saw was in the Castelfidardo accordion museum. I believe it was an Excelsior and was 2 voice. The museum also has a Frontalini bass accordion with a huge horn.)
My conclusion from everything I have seen so far is:
1) If you must have a bass accordion with registers, the Pigini P39/C39 is the only option.
2) If you don't need registers but just want the best bass sound in a new bass accordion, the Bugari is the one to get.