Measure the nut to nut distance of the hub with the 5 speed freewheel. If it's 126mm, then it's likely that somebody replaced a six speed with a 5 speed freewheel for some reason. If it's 126mm, it's probably spaced for a six speed. Seven speed freewheels had pretty muich the same spacing as the ultra-six speed mentioned earlier. The result was that the seven speed freewheel occupied about the same space as a standard spaced six speed freewheel.

That's a long way around to this: if the hub is spaced at 126mm, you can probably screw on a seven speed and go riding down the road. If it's 120mm, you're stuck with 5 speeds or a 6 speed ultra, probably from ebay.

Or, if it's 120 mm, you can acquire the axle for 126 mm, 6 mm of spacers and make your old hub 126mm. (NB: not all axles are threaded the same and axles used on the same brand hub may be different) You can get something like 2 2 mm spacers and 2 1 mm spacers to make up the 6 mm and then experiment until you get the spacing right. You might need to redish your wheel depending on how the spacing works out and if it bothers you to have your rim a couple mm off center.

I replaced the axle on an old Tipo hub to go from 120 mm five speed to 126 mm seven speed and it worked out fine though I did have to experiment a little with spacer distribution to get clearance for a 14 tooth small cog. Replaced the axle before I built the wheel so re-dishing was not an issue.