RAF Tristars

[Compass Call]

The RAF Tristars had two AAR units fitted in the rear fuselage.
Can anybody tell me what their designation was and who made them please.
What was the reason for fitting two?

CC

[DaveReidUK]

Flight Refuelling (aka Cobham) Mk 17T HDU.

One for use, the other as standby.

[Compass Call]

Thank you:ok: