That is a six speed Uniglide...it requires one chain whip to hold the main body and another to remove the small cog which comes off counter clockwise.

The new mechanics I work with are always baffled when they see these and I actually save old Uniglide cassettes because every once in a while, someone needs one.

Finding a replacement will be pretty difficult unless you have a co-op like ours and an old geezer like me that saves all these odd bits.