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Eldartford is correct. The original ST70 amp contained front panel sockets that you could plug the preamp into. It also had one of the socket holes marked on each side to show where to insert a voltage meter probe to check the bias of the output tubes.
Here is a link to a picture where you can see the original configuration: http://www.tnt-audio.com/ampli/dynaco2e.html
The picture reminds me that there are two of these sockets. That's because although this power amp is stereo, most preamps, including Dyna Kit's were monophonic, so you had to use two. Having two volume controls was a pain. I modified two Heathkit preamps to a stacked configuration and installed a dual pot in one with the signal from the other brought up to it and back.
Generally my rule of thumb is that whenever I find a socket and I am unsure of its purpose, I find it most logical to stick my finger in it. If I get a shock, I figure it is probably important. I try to teach my kids this practical philosophy as well. Do be careful though, with certain sized sockets it is important not to stick your finger in too far, as it might get stuck - this would be a bad thing!