Reformulation of Hamilton's rule
Tracing it backwards this was the earliest reference I found via google scholar, it's from 1988 and uses the $r B C$ notation in the format we are used to.


  Hamilton's rule states that for a social
  action to be favored under natural selection, rb - c > 0, where c is the cost to the
  actor in terms of the effect on (usually a reduction in) his expected number of
  future offspring, bis the benefit to the recipient in terms of the effect on (usually
  an increase in) his expected number of future offspring, and r is the relatedness of
  the actor to the recipient


However, I've traced back a bit further by going through the references and found this Michod paper from 1978 where they defined terms $b_i$ and $c_i$ as the 


  "fitness effect of genotype $i$'s behaviour on others" 


and,


  "fitness effect of genotype $i$'s behaviour on self" 


respectively, which was the first such notation I could find which could be interpreted as $b$ = benefit via the increased fitness of others and $c$ as a cost to self.

As far as I can tell this leaves a 14 year gap between Hamilton's long version and the beginnings of the current notation by Michod in 1978, with the first "rB - c > 0" type of notation appearing in 1988. Obviously my search is not exhaustive - many papers from this period will be hard to find and text search (e.g. using "rB" as a search term) - but it's a good start. If I find an older reference any time I will update.