Interesting and I obviously haven't come across this type before. That must be one of the most recent 7-speed Shimano's and, indeed, there does appear to be a lockring but it's so thin I wonder if any hook spanner can get adequate purchase to remove it. An air hammer? How?

BTW, do the cogs all slip on and off like a cassette and are they unique to the freewheel or will they also fit on a freehub body?