This is the Tamiya TEU-105BK Electronic Speed Control.
Tamiya TEU-105BK Features:
Royal mail at £3.95, or Express at £6.95.
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Yes, this will replace an older mechanical speed controller (and the servo).
Answered by: Ben @ Wheelspin Models
Input voltage on these is listed as 6.6-7.2v, so 8.4v isn't officially recommended.
Something like HW30120201 however would be fine on 8.4v and should fulfill your needs.
Answered by: Ben @ Wheelspin Models