I'm in Australia, so these are Australian model names:
Kawasaki sold the Z200 single in the late 1970s.
The Kawasaki Z250 came in three models around late 1970s early 1980s:
Z250A twin with disk brakes
Z250B twin with drum brakes
Z250C single with drum brakes
Kawasaki sold a BR250 single in the late 1980s.
Yamaha had the SR175 and SR250 singles, both were a bit cruiser style.
Honda sold the CR250RS from about 1980 onwards, then the CBX250 with much the same motor but very different styling. The motor was essentially the XL250S motor. The CB250T and N models, also known as Superhawks in some places were twins, as were the CB200 and CB175 of the mid to late 1970s.
Suzuki's GN250 was similar to the Yamaha SR250.