This is the Anti-Mass Spectrometer from Half-Life:
So where are the Astro Detainer's arms? They're not there.
Here's a close-up of the AMS' emitters, which in the game seem to just float freely in mid-air:
That's the way they looked in 1998. However, here's the same part of the device, as seen in the official fan remake, Black Mesa:
You can just spot the arms in question holding up the redesigned emitters. Once you know they're there, it's easy to spot them even when the device is in motion.
