they're not very nice to work with, either leave it to shut down & walk back into the rm & when it transmits it will clear the tamper,
or view the signal strength in an eng menu
or delete the tx & put it back on
they get unstable with age
Looks like a thin sprayed coating
I recal recall aritech ones filled with resin
Dragon ? with thin coating & ADE one with a even thiner coating
Google images have very few pic of bell mods
never seen anything or heard of this co. before, however
01297445 REVDUN ELECTRONICS LIMITED Dissolved 22/05/1993
01907384 REVDUN SECURITY PRODUCTS LTD. Dissolved 06/11/1990
Google throws up your topic on here about a panel & a linkdin page which say they where in Birmingham ?