BBC Model B+128K
The BBC Model B +128K was the final model, it squeezed every last bit of memory out of an 8-bit computer. It was launched in 1984 shortly after the BBC Model B+64K and was soon replaced by the BBC Master.
The "Tube" was an expansion port which was designed to connect other processors to the BBC (6502, Z80, 68000 or ARM 1 RISC). Interface cards were specially designed for the tube. For instance another 6502 with 64K of RAM or a Z80 cpu also with 64K of RAM and CP/M 2.2 as operating system. Acorn used the name CPN for CP/M obviously for a trademark reason.