Looks very much like it was designed for a riser. I think I have a system that would accept a shorter version of this board, it's a very low profile 486SLC (also 16b bus like 286) system with a short riser with two slots, one on each side.

I'd expect the holes to be filled if it was shipped without those slots. I think due to the wave soldering used back then but not sure?

Here is a picture of my 286 which has only 8 bit slots in 2 places, even though it could support 16 bit. The 16 bit segments have closed holes as they were never fitted.

Ah I'll now dig out my riser box because this is my first 286 with just one ISA and I also think it's made this way because the solder pads look to clean from factory instead of someone removing them for parts

Also you can see there is battery leakage and I'll have to use vinegar though I'm concerned about the chip above where I removed the battery, all the legs are green up to the silicone could the vinegar travel into the chip and damage it?

I've seen some of my boards here the same deano but they are 386 and newer I think even my only other 286 has filled holes so maybe just some manufacturers were different and left them unsoldered