Andy, somewhere, sometime, I vaguely remember seeing or reading something about this, and from what I can remember, it was something to do with one or more of the various ancillary "bits" fitted, and their positions, and possibly the actual engine fit. The B.35 has them as depicted on the resin parts, the B.IX / PR.IX has similar scoops well to the rear of the side cowl on each side, the B.XVI evidently didn't have them, and so on.
Anyway, they've now been successfully removed from both sides of both resin cowlings, and the areas sanded and cleaned up. The cowling joint lines have also been re-engraved where required, and work is now underway on the wing tanks.
Although strangely not shown in the instructions, the Tamiya kit has the smaller ( 50 Gal ? ) tanks included, but in general, the B.XVI was fitted with the larger, 100 Gal tanks, and PF510 would certainly have had these for this "Op" to Schleizheim / Munich. These larger tanks can also be seen in the photos of the Keil Op., shown earlier in this thread. They were very often left in "aluminium" dope, but many photos of LNSF Mossies, including the Keil pics, show them in the underside colour (Medium Sea Grey ), sometimes, as in the Keil pics again, with the exposed upper surface in the top-side camouflage colours.
I'm using the larger tanks left over from the strange Airfix B/PR.XVI kit and, like the rest of that kit, the tanks are far from perfect !
Fit is fairly poor, with a lot of sanding required to remove the seam lines, but the main thing is, the shape of the "nose" profile is far to pointed and is also "flat" on the top surface.
So far, this has been improved, as far as possible, by filing a more "rounded" profile and, once in place and fully set, I'm hoping that I can make further improvements by "building up" the shape with "Milliput", and sanding this to at least resemble the actual shape of the tanks. I'm not expecting them to be perfect, but at least it should improve the overall appearance.
Pics below show the work to date - not a lot, considering the hours spent.
Pic 1. Those "scoops" removed, and cowling panel lines re-engraved.
Pic 2. The Airfix tank half (top), compared to the Tamiya kit part. The Tamiya part, although smaller than the Airfix example, seems to me to be somewhere between the size of the actual 50 and 100 gallon tanks.
Pic 3. The tank on the left has been filed to a more rounded shape around the nose profile. Compare with the untouched Airfix tank on the right.
Pics 4 and 5. The first ( port ) tank in place. Once fully set, the nose profile will ( hopefully ) be improved by building-up with "Milliput" and sanding to shape, and the tank to wing joints will also be filled and tidied, before adding detais such as the test and drain cocks etc.
With luck, I should get the other tank done, and the work completed on these, some time tomorrow, an then fir the wing tips, before moving on to the fuselage and interior construction..