Since it's receive only, I would imagine that impedance matching isn't as critical. As it's a solid connector, there's no shield or center conductor, it's one big chunk - so it's not regular SMA or reverse SMA at all. I don't know if SMA will thread onto it it, but using regular SMA stuff would not use the center conductor and solely use the outside conductor as the antenna if it even has one, hence you'll need to carefully select what you want to connect to it.
That webpage suggests a BNC mod. I'd be tempted to do it.
I sort of have the same issue with getting an antenna for a Uniden CB HT, it too does not have a center conductor for its antenna and I have to choose accordingly. It happens to be 5/16" thick instead, and I think NF threads... Maybe I should BNC mod this CB...