Ant T - I'm pretty sure your S-61 ELS is the same as our Sea King one. As far as I can remember, the ELS mod was definitely a post-production issue, there were a number of MRGB oil leaks which resulted in ditchings when the SK was in first in service with the RN.
Since operating a helicopter in the maritime environment with so many pipes external to the MRGB that could split and pour all the oil overboard is not so clever, the ELS mod was born.
What I don't know is if it was a Sikorsky produced mod or if it was Westlands producing it under licence because they built the brit mil Sea Kings.
Perhaps the history of the S61 MRGB was what drove Sikorsky to try to produce a MRGB without external pipes (the S-92) so that such failures were removed for ever - unfortunately that came back to bite them in the a*se as they invented a whole new type of failure.