BSA - Back Slot Adapter (Yamaha CA-01 remake)

Definitely not, completely different connector on the computer side. An adaptation on MSX Flex to connect to Extension Bus could be possible, but Alexey‘s focus seems to be with the, admittedly very nice, Yamaha range. But surely there are more HX-10’s in the wild than CX5’s and derivatives... So could still be interesting...

If someone could confirm that HX-10's expansion bus pinout has the same pin order as in the MSX slot and show where the power and ground pins are located, I could verify whether FLEX is suitable for that Toshiba's MSX. So far I could verify that SVI-728's expansion bus connector is compatible with FLEX, but you will need to remove the key on the IDC connector to be able to plug it in.

Pencioner told me it is incompatible as it is. According to the Hans Otten page the pin out is the same as Spectravideo (as in the same signal on same PIN number) but the pins may be on different physical location (so probably top-bottom switched). This would mean the cable would need to twist each pairs (easiest to do on the MSX cartridge slot side of Flex I guess). But Pencioner should confirm it is top-bottom switched or something else.

17-32 : A0-A15
33-40 : D0-D7

That doesn't really help. The order is still unknown. But so far I can see that the expansion bus and the MSX slot's layout are identical. If I can use FLEX with SVI-728 then it's likely that I can use it with Toshiba.

Back to the topic. I still can sell 5 BSA modules with or without cases: 15 Euro without case, 35 Euro with case.

It would surprise me if an MSX bus with the odd and even pins reversed exists, even the Samsung SPC-800 has the same pinout. (I thought it was different)

The only differences I've found is the polarizing key and some pins that sometimes are not connected (Sound In, +/-12V for example), and rarely there is a reserved pin that is used to add a primary slot signal.