Hi,
recently I started the restoration of an IBM 5150 in my collection and while checking out the motherboard, I stumbled upon some RAM chips that have absolutely no marking or label at all (neither on top nor on the back). Also looking at them under the microscope shows no trace of deteriorated or just badly visible markings.

I have the 64KB-256KB motherboard version, so all banks should be populated with 4164-type RAM chips to reach the 256KB, and also Switch 1 and 2 were configured for 256KB.

First I thought those might be dummy or placeholder chips (is that even existing?), but after successfully bringing the 5150 up to live again, PC DOS 3.3 shows 216848 free bytes out of 262144 bytes total memory. There is also no 201 POST error, unless I remove one of those unmarked chips on purpose. Then I get the correct 201 error indicating the location of the removed chip together with a parity error. That let's me assume that those are indeed working RAM chips.

Has anyone of you ever seen those completely unmarked memory chips? Was that common at that time? Did they have a special purpose?
And moreover, is there any more thorough RAM test program for the 5150 existing?
I would be really curious to find out more about those chips!