I'm pretty sure this is a 7-speed bike. You can verify that by putting the right (rear) shift lever in its fully-forward position, and slowly pulling out back until it stops. The number of clicks you feel will be one less than the number of gears the shifter was designed for.
If it's a 7 speed, (6 clicks,) the rear triangle is almost certainly 126 mm wide. (Some bikes in the years of 7 to 8-speed transition were set at 128.5 so that both would fit.)
The good news is that it doesn't really matter. You can use a 130 hub in a 126 steel frame all day long... the bike will outlive you. It'll be a bit harder to get the wheel into the dropouts by the side of the road, but we're only taking about 2 mm per side. A 130 rear wheel lets you use any of 7/8/9/10-speed cassettes, so long as your shifters match your cogs (or you friction shift) and you use the right chain.
Basically, any road wheel made from 1980 to last Thursday will probably work.
--Shannon