The first one I bought when they first hit the US market and others I saw from that time did not have belt clips. The belt clips from older style vista lights fit, as they all are built to fit onto the typical reflector mounting brackets.

The new one I just bought has a nice belt clip included.

The best way the make these lights work anyway is to mount them to the bike so they are horizontal all the time. The belt clip is not as good because it can angle the light up or down or sideways and you lose most of the visibility or brightness when it is tipped.
It looks like a lot more than a 50% loss.

You can make a belt clip easily with the perforated strapping they use for pipe hangers, sold in hardware stores. It's cheap, cover the metal clip with black tape to protect the clothes. Triple or quadruple the layers of the strapping for strength.