This thread will help identify your bike and the differences between models
https://www.xs650.com/threads/xs650...workshop-manuals-and-other-information.30569/
probably need this one for your gauges or if you replace those.
https://www.xs650.com/threads/gauge...0-83-gauge-repair-honda-70-71-750-4-ko.52013/
A few variations on tanks. When looking through the ID thread, (above), you can see the differences in the Tanks, have also explained on most models.
Any tank from 74-83 will swap straight onto your bike.
Standard tanks are larger than Special tanks
74-79 Standard tanks are 15liter and have 2 petcocks,
74/75 tanks gas cap are hard to find replacement for and if you do they are very expensive o don't buy a 74/75 tank with a broken or missing gas cap.
76/77 Standard tanks don't have badges and a replacement gas caps can be bought after market. Petcocks are manual
78/79 Standard tank have badges. Petcocks are Vacuum operated. Same gas cap a 76/77
78/79 Special tank have 12.5 liter with 2 Vacuum operated petcocks. Different Gas cap looks similar to 76-79 Standard tank and can be bought aftermarket. All Special tanks have the same gas cap
80-83 Special tank are 12.5 liter, with a single vacuum operated Petcock