Actually I have seen 3 types of the BTC 5100 and owned 2 of them. Some older version also had doubleshots keycaps.
First was foam and foil. Foam and foils have a real pcb, then little pieces of foam wrapped in foil, then a plastic slider with MX compatible then a spring or rubber cup under the keycap.
Second, was rubber capacitive where they used the same pcb as before with little individual rubberdomes with carbon? contacts on the bottom then same MX compatible sliders. I liked this keyboard quite a lot before I discovered MX. It had a pretty good feeling and was reliable for many years before it just stopped working one day. Probably one of closest things you can get to Topre feeling without paying more than $60.
Third was last version with a cheap mylar membrane and full sheet rubberdomes mat, MX compatible slider. This one felt better than a lot of domes, but still you knew it was a domes board and was way crappier feeling than the older version I had. After disappointment with that one I went MX.