I have a good Sylvania Rebuilt 21FJp22. The safety glass has deep scratches. If I remove it does it now become a 21FBp22 and I can use gasket style FB safety glass?
Thank you,
Perry

FB is the gasket style CRT part number and FJ is the bonded.

I have not been able to find envelope differences between bonded and gasketed round color CRTs. I have swapped gasketed safety glass onto bonded tubes post cataract removal (either do to safety glass scratches or cracking of the safety glass during cataract removal) successfully. It is absolutely doable. The new safety glass may add a fraction of an inch to CRT dimensions so be careful not to overtighten mounting screws.

Sylvania spec sheet says

MECHANICAL DATA
.................................................. ....21FJP22...........21CYP22A
.................................................. ....21FKP22..........21FBP22
Minimum Useful Screen Dimensions
(Maximum Assured)..........................19 ¼x15 ½........19 ¼ x 15 ½ Inches
Minimum Useful Screen Area..............261.................261 Sq. Inches
Overall Length...............................25 9/32 ± 3/8..25 1/32 ± 3/8 Inches
Diameter.......................................... 21 Max............20 13/16 ± 1/8 Inches
Anode Terminal (Recessed Small
Cavity)........................................... ..Jl-21................Jl-21
Base (Small Shell Neodiheptal 12-Pin)..B12-131...........B12-131
Basing............................................ .14AU.................14AU for 21FBP22
.................................................. .............................14AL for 21CYP22A
Operation Position.............................Tube Axis Horizontal
(Base Pin 12 and V Grooved Panel Pad on Top)
Weight (Approx.)...............................43........ .............36 ½ Pounds

There is no difference between a 21FJ or a 21FB itself. The FJ has a bonded faceplate which means there is a resin that was poured between the faceplate and the tube itself. Removing the bonded faceplate will make it a 21FB and then you can find a faceplate and use a gasket. That's all there is to it.

Sylvania also says the FJP and FKP aren't interchangeable. Why?

Sylvania also says the FJP and FKP aren't interchangeable. Why?

EDit:
Can you point to the reference for that? I can't find it on the spec sheet I have.

Is it just that the FJ has anti-reflection coating and the FK (apparently) does not?

I have never seen a 21FKP and if the anti-glare is all there is to it, It will work the same. A GLASS 21" ROUNDIE PRETTY MUCH WILL WORK IN ANY APP to include replacing a AXP metal. I have used 21FB & J's in place of a 21CY & AX with no problem and good results. FK? Never heard of it or ever saw one. If it was only an anti glare feature, it may have been very late in the game and very few were produced as a selling point replacement. I never saw a production set (or tube by itself) with one and I saw hundreds of roundies when I worked after school in a tube rebuilding plant here in Knoxville, Tn.

PS: ALL 21" roundies use the SAME gun mount. Glass and Metal

I thing some early Zeniths used the FK version

Early Magnavox roundies had the anti-glare also.