Agreed with James on the likely detector identity. The pneumatic controls on the front panel should positively ID it, though. I think I've only had one 3400 pass through my hands that even had a detector in it, and that was an ECD(all the others were set up to work with things like the Finnigan Incos or TSQ.
With that said, I'd be leery of going down the path of a 3400. It's a bit newer of a design than the HP 5890, but Varian GCs never sold in the same quantities as HPs and there are fewer folks who know them now. Also, if you want to run it with a computer(as opposed to a strip chart/integrator), you need the LAN card which can be a bear to find it not present(unlike the HPIB card for the 5890). There again, I've never been too deep in Varian GCs outside them being MS front ends, but I don't remember them even having a good "external control loop" system like the HPs(where you could tie the GC, integrator, and autosampler together and program them using INET or HPIL), even though I'd not be too inclined to run anything short of a computer these days. There are options for simple data handling these days through an A/D converter(some folks even use a sound card) but of course you lose full instrument integration.