Condition I've got it in. It looked much whiter in the pic the seller sent me. At least it works save for two contact pads on the rubber domes not being flat enough to make a contact confidently making the affected keys feel "membrane"-y (can be fixed either from a donor keyboard or by sanding them to flat), and a broken scroll lock LED.
So much dirt and dust in the keyboard, hopefully nothing some soapy water can't get rid off.
PCB board in it's original condition. The partly snipped off resistor doesn't appear to affect operation, better to leave as is.
Closer image of the board when I swapped the loose domes to less affect keys I frequently use. Some of the keys have alternative contact pads at interesting locations, probably for differently stabilized keys.
Rubber domes from the keyboard. The loose gray one is from the spacebar, it feels a bit heavier loose than the green ones, and seems seems it's both from a different mold and is thicker, but the effect is less pronounced in action, likely due to the heavier weight of the space key.
Keyboard shown reassembled. I've lubed the sliders with Mannol MP2 multipurpose automotive lithium-grease. It doesn't seem to eat the plastic or the rubber domes in my BTC 5139, however it seems there's a break-in period while the keys feel a bit scratchier than originally and uncleaned, but enhances the sound of the keys. Might be due to the amount of grease I've used. Also worked well on the stabilizers save for the spacebar one, which is just so clumsy.
In a few weeks I might be able to borrow a DSLR camera and add the pictures of the keyboard to the Wiki, but that will depend on the current pandemic. If not then I'll use my phone camera in daylight to take a few extra ones. I need to disassemble it yet again to fix a few domes and maybe the LED anyways.
EDIT: Used the wrong way to share images from imgur.
