Still it remains unclear for me whether the mentioned ISB is mandatory for the AVRO RJ or not.
In line #1 of the document "BAe 146 SERIES/AVRO 146‑RJ SERIES AIRCRAFT" are adressed and in in line #3 it says the action is "mandatory" but in section "A" it says "Aircraft affected by this inspection are ... ALL BAe 146 100, 200, 300 Series". I might be not the sharpest knife in the drawer but it's still not clear for me.
The only way to verify what aircraft the service bulletin is applicable too is to read item 1. Planning Information, A. Effectivity section.
The AD mentions only BAe 146 series aircraft.