What bike had a tank almost identical to a 650 Maxim?

I've acquired a tank that I thought was from a 650 Maxim. It looks like a direct fit, and the shape of bottom of the tank is also a match. However, it's about 1/8" narrower between the mounting "cups" and that's just enough for it to not quite fit.

It appears to have been red originally, but the part number was filled when it was repainted (not by me, thankfully).

What bike could this have come from? A 400 Maxim maybe?

 

I believe the 550 tank was slightly smaller and only fits the 550.
The 750 tank will fit and add extra capacity, don't know if the 700 tank will fit.

 

Thanks, O 'n B. It's definitely not a 550 tank. To be clear, I'm not trying to find a tank to fit a 650, I'm trying to i.d. the tank I have. It appears to be identical to a 650 tank, including capacity, except for being slightly narrower between the tank mounts.