I don't believe different tanks were ever used (i.e. one size port and a different size starboard). That said, there were a number of "generations" of fuel tanks and so if one was replaced I could see where it might be one of the different generation/sizes. Between looking at varying years of 22's when shopping, and then doing some research prior to replacing my fuel tanks in 2002, here is what I believe I know about the generations of tanks in the 22:
1) 1990's to around 1999:
18 gallon Moeller tanks. These are simple rectangles and are not translucent. They are held down by individual metal "clips" that slot into a recess in the sidewall of the tank.
I think during part of this time there was also an optional 5x gallon single aluminum tank available that straddled the center area.
2) ? to 2002
25 gallon Wayland(er?) Marine tanks (hard to read label; company was in Burlington, Washington). These are "sculptured" tanks and have slopes on the bottom, outboard side, and top to match the C-Dory shape.
3) 2003 (only?)
20 gallon tanks, maybe Moeller. I don't know much about these. I think they had has some problems with the Wayland/er tanks and so had gone to these but people then wanted larger tanks (guess on my part based on comments read at various times).
4) 2004 to 2005 or early 2006
25 gallon Moeller tanks, part numbers FT2536 and FT2537 (one is port, one is starboard).
5) 2006 to at least 2012, likely to present
23 gallon Moeller tanks, part numbers FT 2339 (port) and FT 2340 (starboard). These are sculptured and just like the previous tanks except for one major change order which was "cut down" by 1" in height" in February of 2006 (there were a few other minor changes at various times between 2004 and 2006, but they were things like rotate pickup, change size of barb, etc.).
My theory on this change is that when they went to the flat cockpit sole "liner" in 2006, that liner includes a molding for the fuel tank bay and cleats. I'm guessing that necessitated the tanks being slightly shorter to get them in and out (and maybe for them to fit). That 1" removed reduced the capacity by 2 gallons on each tank.
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When I went to buy new tanks, I wanted to keep the 25 gallon capacity if possible (I had the Wayland/er tanks). I contacted Moeller with the part numbers for the 25-gallon tanks and inquired about buying them. What they told me was that they did not own the rights to make those, and it seemed that the person who did (can't remember if I got name or not) was not someone they/I could get ahold of. I suspect it is someone who was associated with C-Dory back in the 2005/2006 era but is no longer. Since the new boats need the 23 gallon tanks (I think) to fit into the lined tank area, the new builders have no reason to want to sort this out and have rights to the 25-gallon tanks.
Moeller can make the 23-gallon tanks for anyone. So they can be ordered through any Moeller dealer. I purchased a pair of these, and have made alternate use of the "extra" space above them vs. the 25-ers.
So anyway, I'm thinking the boat you are looking at could possibly have had a tank replaced, and at that time was a different "era" of tanks than the other, non-replaced tank.
I have the opposite problem as you: always starboard heavy.
Sunbeam