Gus, I agree Donny has to be right: no sub ever had (nor ever will, IMHO) control surfaces sticking out on one side only!
Now, if we accept that as the only thing we do know for sure, then - like Sherlock Holmes - we can deduce the remaining possibilities, however unlikely, must contain the truth - i.e.: (cue horror music) the instructions could be wrong! Seriously, not only could the instructions be wrong, but even the parts depicting the fins themselves could very likely be wrong - we've certainly seen both, before.
The case that immediately comes to mind is an Asian manufacturer (I can't remember which kit, now) clearly lacking any drawing(s) or experience of the parts in quesiton, got them wrong due - quite obviously and very predictably - to a verbal error in the description by someone literally "phoning it in"! Seriously - actually, it was most probably a written error in translation.
Now, regarding your 206A kit - after decades of offshoring jobs - probably an entire "We Are The World" of German-, English-, Korean- and/or Chinese-speaking technicians were involved - none of whom knew anything about subs whatsoever - much less the 206A, in particular - so how likely is it, then, that mistakes in the instructions - even in the shape(s) of the fins themselves - could have been made, and gone uncorrected? IMHO the answer is: "Very."
In fact, the whole "One curved up, one curved down, pick which one" instructions you describe sounds like a collossal misunderstanding of (again, some linguistic description of) articulated fin-tips - similar to the hinged wingtips on the XB-70 Valkyrie bomber (but of course both always deployed together).
That such gross errors can go undetected (or at least, uncorrected) is definitively proven by the Heller 1/400 TypeVII U-boat, whose limber holes, otherwise accurate and detailed, are molded everted - protruding as little bumps - instead of inverted, as (obviously) intended. Someone(s) crafting the injection mold no doubt f-ed up, and neither found nor fixed the mistake - and neither did management, possibly also deciding it was too late/expensive to go back and fix.
So my assessment is definitely that the Revell 206A instructions and shape of (at least one of) its forward planes must be wrong - and I'm sticking to it. (Unless, of course, anybody wants to go with my "opposing torque/slipstream" explanation, which was a piece of sheer genius, whether true or not! )
Either way, though, both foreplanes have to rig out - or in - together. The easy solution: stow them both in, so you don't have to explain anything!