Looks like a TTS (Tormach Tooling System) copy.

cnczone is the place for ATC threads, although for that size of mill, most seem to revolve around the TTS system.
The TTS uses a collet type system that clamps onto a parallel shank, which seems fine provided you're not pushing the spindle too hard especially on interupted cuts.
Without going to a full spindle redesign/rebuild, the other option is a suitable reverse thrust setup that when you unscrew the drawbar the tool gets pushed out, and combined with a small airgun that engages to slacken/tighten the drawbar.
If you do fancy the full spindle rebuild option, you can't really beat this thread for ideas/info - BT30 spindle from scratch - with power drawbar and ATC of course
Personally, I'm just waiting for an ATC spindle to be delivered (if it ever does get delivered!), then I might think about what machine to put it on...