Thanks fobotcso and 86.
For both of you - the Com port cables are both standard fittings on their I/O cards, not feeder cables from the Mobo itself. There is a mouse pin outlet on the board but I have this disabled in the Bios. The Mobo is quite old - no manufacturer's name that I can see and it's fitted with a Socket 7 carrying a Cyrix P166 equivalent.
fobotcso - see above. The cards are standard 16550 AF UART jobs and don't seem to require drivers - at least, not for DOS 5/6 and/or W95. I'm only using one but it's a standard EISA fitting and there are spare EISA slots on the board so it would take two if necessary.
Presumably you are looking to use two 3 1/2 drives, not one 3 1/2 and one 5 1/4. One alternative, (slightly expensive) is to fit an Imation 120 drive. will read and write to both 120 Mb disks AND standard floppies. Comes with drivers for 95/98 and NT so might be suitable.