Hi! Judging by the source, I would say it's very badly written in Spanish so I guess "junta" may not be the most accurate term for that. "Junta" is generally translated as "gasket" but it is not exactly what the picture portrays. A "gasket" or "junta" in Spanish is generally something used in between two things to prevent air flowing/debris accumulation (generally in engines/pistons), etc.

I would say it's just a "cover" for the brush of the vaccum cleaner.

Let me know if I can help you more!