Yes indeed - there was never a case of mixed engine types on the same air-frame.
Also a 535C (as built and fitted) aircraft was never temporarily fitted with E4 engines - They only had a full re-engine if the carrier or lessor wanted such, as has been noted above with the various fleet listings.
A new leased Monarch air-frame G-DRJC was not built with ETOPS and MON/OM wanted to do an ETOPS fit on her, but the lessor refused so an older ex 535C air-frame G-MONB had to chosen to have the ETOPS fit.
A quote from a chap (Red 5) some years ago here on Pprune who was a MON engineer -
NB was not the first choice, G-DRJC was the logical choice but the owner ILFC did not want it altered, therefore as the company had already committed to a very heavy Orlando programme and needed six aircraft NB reluctantly became the next aircraft to be converted.
The reason the company did not want to convert NB to ETOPS standard was due to the fact that being such an earlier build aircraft, the amount of modifications involved including major changes to the equipment cooling system was far more extensive than later build aircraft. However the task was completed and it has been a far better aircraft compared to NC.
G-MONB had a special status among British registered 757's.
It's old when she retired, & was the first of a type that really transformed the reputation of the holiday charter airlines and different mission requirements.
Quote from Tempsford -
The move to take the B757 was a major contributor to Monarch's survival in a very competitive market (1983) and a very shrewd one made when they still had Bristol Britannia's !.
MON/OM were also in the leading pack regarding EROPS (1988). Overcoming the ''if it ain't got four engines'' school of thought was hard work, but look at the situation now, few even bat an eyelid when they board a two engine aircraft for a long flight (few know what EROPS is)
Monarch converted 6 757 to ER config G-MONB, NJ, NE, NK, KE, & JB
Quote from Brakedwell -
I flew G-BKRM at Moses Lake on the 31st March 1983, the first time it had been flown by Air Europe pilots. We returned to UK with it on the 6th of April.
G-BIKF was operated by AE during the 1983 summer season.
Great memories of possibly my favourite airliner. The irreplaceable 757.