Left one is 1980 PE175/250/400, the other one is obviously PE but i wouldnt like to buy it and discover the hidden surprises under that horrible yellow paint. For your information, all the plastic PE tanks are interchangeable with each other even though capacities vary, the mounts are the same. Rough rule of thumb capacities vary from 12 L for 78, 11.5 for 79 and 10.6 for 80-84, but its no definite ruling i have had some late model tanks that i know are the original ones shipped on the bike from factory and they have measured much more than the factory spec of 10.6 liter. I am not the only one who has discovered this either. If you look underneath the tank there is a date moulded into the tank when it was made. This date does not indicate what model bike its off though. It simply says when it was made.
