The very first, not commercially available, must have been an early prototype
while developing the MCA bus and protocol itself, maybe a bus attached RAM or ROM to develop and test the different transfer modes.
I'm speaking of a commercial adapter. An IIBM PC didn't need drives to have basic (no pun intended) functionality, but a display adapter was needed (and there were options for different levels, with the adapter setting what was used). In contrast to that,
the system unit of a PS/2 didn't *require* any adapter to be installed to function. The development of microchannel also meant a big lead for IBM to initially offer adapters that enhanced the systems.
The 8515/A and MME adapters would have had to have been prototyped that the design would work work as the systems were announced - but I'm thinking it was the 4Mbps Token-Ring adapter (which adds networking, and it was from IBM, so had to Token-Ring;
another contender was the baseband adapters like on the PC). The 'Adapter ID' was E000h. However, the 4Mbps "Long" Token-Ring adapter didn't change at all over time (even keeping n "equal flow" griding on the PCB like a prototype board); the same
microcode version was just soldered in for the ROM, as visible in this two adapters (the top adapter was produced late in 1986, and the bottom one almost two years later - Token-Ring was announced at IBM on October 15th, 1985):
https://ibmmuseum.com/Adapters/Network/Token-Ring/IBM/Microchannel/4Mb-Long/MCA4MTR.jpg
In contrast, the "Long" 8-bit Token-Ring adapter had different microcode versions and always had socketed ROMs - and lost the "equalflow" griding on the PCB over the same two-year timespan:
https://ibmmuseum.com/Adapters/Network/Token-Ring/IBM/8-Bit/4Mbps/8Bt4MTR.jpg
Both the 8-bit and microchannel "Long" Token-Ring adapters were replaced with a "Short" version, and with time, the 16/4 (E001h) adapters...
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