Had one of those for many years. '69 to about 2004... Well, what I had was a DB-200, functionally the same with a different nameplate. The spring on the headstock is a return spring for when you need to move the spindle. It will feel loose, I never noticed if it would spin around. As a drill press is the most common use but there are other uses. Once you get to know the machine, there are many "uncommon" things it will do. I didn't get the riser post or the bracket when I bought mine. "Petty Officer Hall" made one for me, what he felt like it should look like. I stumbled across one some 20 years later... and snapped it up. Eventually acquired another machine. But no book, blah. Heard of a fellow once who used it as a hand drill. But I already had a hand drill so never tried it.
Cutting tapers was one of the most useful jobs to me. Just pull the pin and swivel the headstock. I had one taper to cut regularly so drilled a second hole in the frame to set the pin back in. The power feed is a good accessory, for cutting. Not worth a da@@ for threading. That's why it got replaced with a larger machine. Then the 6X12s (Chinese) came along and things started getting really crazy.
Mine came from a pawn(?) shop in California in '69. No box, no manual, no tooling. I had to figure out everything for myself. In those days, there wasn't much understanding of the difference between M6X1 fasteners and 1/4-20 &-28 we had on the ship. Not even the ship's machinist had any ideas in those days. But I learned a little here and some more there and eventually had a usable machine.
Something to take note of; There are several small 3 jaw lathe chucks around. The UniMat has M12X1 threads on both the headstock and the tailstock. Some of the parts floating around have M13 or M14 threads. It is NOT 1/2-20, although to an untrained eye it sure looks like it. I bought a tap and die in the appropriate size, made a few "accessories"...
I still use the chucks on occasion on the big (to me) machine. I now have a Craftsman 12X36, MT-3 on the headstock. I made an adaptor of sorts, took a MT-3 to Jacobs and cut it down to M12X1 so I could do my small jobs. The smallest most of my chucks will go is 1/8th inch. I have motor shafts and axles in the M2.5 range, about 0.100. So, out come the UniMat chucks, on such a huge machine. It looks preposterous, a 75mm chuck on a 12" lathe. But it works for my small work.
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LOL The manual states when new and just out of the box the spindle is locked up for shipping purposes. You adjust it so that spindle will turn. You are right, the spring is for the return if the lathe is set up as a drill press.