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There are some different ways to look at this situation.
The 21FB's have the seperate glass, when replacements were available you only received the tube and reused the glass front, either mounted away from the tube (flat front) for the later style that followed the curvature of the CRT. For safety reasons with the FB, you should use the curved glass front and it "may" mount up in place of the 21FJ.
The 21FJ's as you know have the bonded glass front.
As for mounting and substituting, by most accounts in the "day" no. But, if you have an FJ without the (bonded) safety glass front then you basically have an FB. Keep in mind that the FJ with the bonded glass has more "material" to it. As for the newer curved glass sets that used an FB, then an FJ may work providing the mounting set-up was the same for either tube.
There are no solid answers here and each situation would have to be looked upon as a case by case basis. It is much easier to take a 21FB and convert it to an earlier set that used a 21CYP22 as the glass is the same size, than trying to go the other way with a "one size fits all answer".
This is why you don't see these tubes as direct substitutes, as the manufacturers had to insure implosion protection was in place.
I hoped I "shed' some light on this without being confusing!
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