That looks like 6-speed Uniglide to me but there are only 5 cogs in the photo. With Uniglide, only the top cog threads on, all the others just slide on/off. At least I've never seen a Uniglide cassette where the 2nd cog threads on and I have Uniglide cassettes with 13,14,15, and 17 as 2nd position cogs.
Are there any notches or splines in that smallest cog? It doesn't look like it to me but I can't tell for sure.
If there are notches/splines, then it is not threaded and should just slide off. Perhaps it is dug into the freehub splines a little preventing it from sliding off.
If there are no notches/splines, it might be threaded. Since the threads on the end of the freehub normally don't extend that far, perhaps someone took a top cog and put it in the 2nd position and threaded it way past the end of the threads thus jamming it on for good?
Here are some Uniglide cogs. The 2nd-6th position cogs should have notches/splines on the inside like the cog on the upper left. The threaded top cog is the lower left.
http://www.sheldonbrown.com/k7.html#uniglide