These are differential adjusters for Mk2B heater pcb and borosilicate glass bed surfaces, a companion to my Mk2B Bed Mod project: https://www.thingiverse.com/thing:2477091
The adjusters are in two parts, the lower part has an internal M4 thread, 0.7mm pitch, and an external thread, 1.0mm pitch. The upper part has a matching internal thread, 1.0mm pitch.
So, as the lower part is rotated 360°, it moves down the M4 thread by 0.7mm, but the upper part moves up by 1.0mm. The net movement is thus 1-0.7=0.3mm.
In short, very fine adjustment is possible.
The upper part has a recess for the heater plate, the peg should locate with the hole in the corner of the pcb. The upper walls are designed with some spring in them, the glass will be a tight fit, but the plastic will expand slightly as it warms up.
There are three versions of the upper part here, for 220x220x3.8mm, 220x220x3.0mm and 214x210x3.0mm glass, choose the one appropriate for your glass.
I printed mine form PETG, and I've tested them with bed temperatures up to 80°C with no problems.
I've been using these for several months now, and I'm very happy with the result.
Using PETG is important, PLA will likely deform when it gets hot, and not return to the correct shape.
You'll need 4x M4 bolts, 25mm threaded length, and 4x M4 nuts, 8x if you choose to use the locking rings. Half nuts are best if you can get them, less weight and less height. I prefer stainless...
If you've not seen my 'Mk2B Bed Modification for Anet A8' thing, good time to go look: https://www.thingiverse.com/thing:2477091 - it describes how to use the Mk2B heater pcb.
So you can either drill out the holes at each corner of the 'H' frame to 4mm, or run an M4 tap through, either way works. Insert the M4 bolts from underneath, and screw on the lock rings (if you choose to use them), followed by the corners. Push the heater pcb into the recess, place the glass on top.
Level your bed in the usual way, but remember - turn the adjuster clockwise (when viewed from above) to raise the corner up.