Membrain Ltd was founded in 1972 by Anthony Davies OBE, it manufactured Automatic Test Equipment for electronic systems, evolving through memory (whole board) testers, wire-wrap backplane testers and multiple generations of functional circuit-board testers. Membrain was acquired in 1977 by Schlumberger.
It appears that Membrain bought MUSS around 1976 and ported it to their MB7700 [1][2], which was a machine that was embedded in test equipment. It is an obscure machine for which there seems to be very little information. The machine was functional by the end of 1976, and it was in production equipment by mid-1977.
It seems that the Membrain MB7700 was based on the Intel 3000 series. The Intel 3001 was the program control unit which executed the microcode and drove a set of 8 x Intel 3002 bit-slice ALUs. There might have been a third chip. The microcode included multiply and divide, and synchronous IO to floppy and HDD heads. The minimum configuration was 32KB, but most units in practice had 64KB of DRAM. The board was designed by a contractor in California.
One former Membrain employee who worked for them in Ferndown near Bournemouth from 1978 to 1981 reports a product generation that ran an operating system based on MUSS, this product generation was already established in 1978.
Another former Membrain employee says it was shaped like a desk and you could sit at it and interact with it in a way which was unusual then – a side effect of designing it to support test jigs on top of the desk with IO cabling to the back for the measurement instruments. This employee wrote an interactive screen editor for it in early 1978. He says they also added some of the first floppy disks available and had an electrostatic printer which for the time was amazingly small.
It looks like the equipment was marketed by Schlumberger under the Factron brand, retaining the model number 7700.
Acknowledgements
The information in this section comes from Prof. Gregory Egan, Doug Duke and Tanj Bennett.
Bibliography
- H. Barringer, P.C. Capon & R. Philips, “The Portable Compiling System of MUSS”, Software – Practice and Experience, Vol. 9, pp645-655, 1979
- Derrick Morris & Roland N. Ibbett, “The MU5 Computer System”, Macmillan, 1979
Rob's Old Computers 
I was a comissioning engineer at Ferndown until the Slumberger takeover. I commissioned the very first unit sets and was manning the stand at the launch exhibition. Your writeup is accurate. There was also an analogue test set in that series. Membrain also manufactured analogue computers (all op amps no digits).