An "ultra" freewheel is narrowly spaced freewheel made by a few companies in the 80s. For example an ultra 6 is the width of a standard 5 speed freewheel and an ultra 7 is the width of a standard 6 speed freewheel.

Also to the OP
Be sure to have cogs and freewheels in their own category as they are not interchangeable.
If you want to replace your freewheel with a individual cogs you will need a new hub or a rebuildable freewheel.