Intriguing obsolete(?) PCB manufacture techniques in this old MSX
This appears to be a punched board (the outline and all the holes are punched in a single operation) with conductive liquid used to create vias. This is sometimes called STH (silver through hole) technology. 

If you look at the holes where there is no via you can see that they are relatively rough and quite a bit larger than you might have for a drilled hole. This also helps speed assembly by minimizing accuracy requirements for machines or people. 

Not sure about the solder mask appearance, maybe a second layer is applied (typically by screen printing) just to protect the vias. 

My experience with this technology is that it's not all that reliable, especially in applications where a lot of vibration or shock is possible. It is very cheap per square meter because they can use low cost paper-based laminates and punch all the holes and outlines out of large panels in a single operation taking almost no time (on a cheap press too). Because the crappy cheap laminates are quite brittle (as anyone who has dealt with damaged cracked boards will attest to), they have to be heated in a batch before punching. 

It was not all that uncommon in that era for factories to have quality problems with (what are now ubiquitous) plated through holes such as cracking around the holes, often caused by poor control of the various chemicals and processes involved, but those issues have mostly been vanquished.