I think the timeline goes like this...
Original bell - no strobe at all, complete unit made by Scanny
"Mk II" - space for an optional strobe (cover elongated with a clear lens) which consisted of a separately supplied PCB that slots in (strobe PCB possibly not made by scanny)
"Mk III" - cover still made by scanny?? Possibly not, siren definately made by Elmdene, metal box with built in strobe. Also white models with 'ADT Modern' and Telecom Security branding (most Telecoms dummies). Also Sonitrol / Sentrol?
"Mk IV" - cover made by Elmdene, siren made by Bentel, an Italian division of ADT
Current bell - complete unit made by Elmdene in China, with matching battery dummy, and now also wireless model.