Soviet fighters and 4.12

Originally Posted by Pursuivant

I thought that it translated as "fillet" (i.e., thin strip of material just ahead of the tailplane used to improve stability for bubble canopy planes), like was added to the P-51D-20.

When I think of a "razorback" aircraft I think of a plane like the P-47C or P-47D-10 which had a "greenhouse" (or "lantern") canopy and a very narrow rear fuselage. Arguably, the P-40 had a similar appearance.

Yeah, early P-47's were called razorbacks ... because of the rather sharp edged spine behind the canopy. Bubble canopy equipped P-47's, therefore, weren't razorbacks, just jugs. Planes in general before bubble canopies were the norm so (in the west, anyway) there really wasn't so much a a need to distinguish them from the bubble canopied version as much as identifying the new bubble version itself as something new and different.

Bubble canopies typically hurt directional stability a bit because of the turbulence (and, in the Mustang, additional canopy height) behind them but not much more, really, than going to a larger prop, let's say from a three blade propeller to a four blade, as when Mustang went from Allison to Merlin. The Mustang, having had both mods, drove work in improving directional stability although the Brits had started work on that issue earlier after testing their first (non bubble) four blade Merlin versions (some interesting test parts there).