SP21 (and SP14 - youd need a ruler to be sure which size you have, but it looks like 21) is a series of online and panel mount DC connectors.
Inexpensive, robust, with waterproof caps, theyre handy bits of kit.
Sadly, however, I dont know what purpose this particular connector is serving. It may indeed have been to power a wheelchair lift. A multimeter will tell you if there's any voltage there.