keenerb wrote on 2021-03-23, 00:26:
Jumper blocks labeled IRQ, STP, 2, 3, ADR STOP, A C D E, X21, 282 (partially obscured)
For my, the "IRQ STP" block is obvious. The four courners are labelled 2,3,4,7, which are the four possible IRQ levels the card can be jumpered to. The two central pins are most likely in parallel, and contain the IRQ output signal. You need a jumper to choose which ISA IRQ pin is connected to that output signal.
The "ADR STP" (i guess STP mean "setup") is kind of similar, but not that obvious. Most likely, the central pins (without labelling) are shorted together to the "master chip select". The labelled pins are connected to outputs of an address decoder. As the labels "8,9,A,C,D,E" look like hex digits, this is probably the middle digit of the I/O port the card responds to. The first digit (in classic PC cards) needs to be 2 or 3, so the choices are 280-28F, 290-29F, 2A0-2AF, 2C0-2CF, 2D0-2DF or 2E0-2EF. As 3D0-3DF is occupied by the CGA card, I guess the interpretation with a 2 in the front is much more likely than 380-38F, 390-39F, 3A0-3AF, 3C0-3CF, 3D0-3DF, 3E0-3EF, but both are definitely possible.
The jumpers in the corner should be X21 and 232, to choose the serial port type: X.21 or RS232. X.21 uses a 15-pin connector, just as the socket on the board. In some regions, X.21 was used for circuit-switched data telecommunication at higher speed than telephone modems could (or were allowed to) do. This fits the guess that this board might be intended for intelligent terminal emulation purposes.
I don't think the port at the edge is SCSI: The (up to 32) lines are driven by three octal line drivers 74ALS756, which are open-collector drivers. The "white bars" next to them are the resistors that provide the necessary pull-up. SCSI on the other and is terminated to a "center voltage", and driven using push-pull drivers. The fourth 74ALS756 does not have pull-ups in vincinity, so it seems to not be driving another 8 pins on the port, but it might be used as receiver chips for 8-bit data with pull-ups provided by one of the ASICs.