The main external difference is the lower fork legs,the 78 had a pinch bolt type axle holder on the RHS whilst the 79 had the more familiar rectangular removable cap type held on by 4x 6mm nuts,the bike in the add has 79 forks
,The 2 models were cosmetically identical bar sidecover decal colours & the 78 has a white fuel cap where the 79's is black,the motors had different reed valves & porting & CDI's varied,The front sprocket guard rear mounting tab was mounted differently on both models also
,pretty much pigeon pairs though