The Orion 737-300s which went on to Britannia after the merger were the early non-EFIS versions (and so was one of the Dan-Air 737-300s, G-SCUH) and also -3B1 power aircraft. It amazes me that they ever got to Tel Aviv back in the day, let alone made it back again non-stop.
They were eventually a separate type-rating (-200 and -300/-900) but nothing to stop you flying both - as previous posters have said, the sim checks alternated between the two and there were normally operator-imposed limits on the number of variants and whether you could fly both in the same duty day etc (didn't tend to be an issue on charter as two long sectors was your working day). Britannia got rid of the Orion aircraft pretty quickly, off to Morris Air then into Southwest - some in service until only a year or two ago, so over 30 years old.