When the Holiday Jets were introduced by the late 60's the new 737's (including Britannia's) all came with built in front stairs.

The early 737-130's that LH used for Condor had both front, and the drop down rear door air stairs.
Britannia and most other Charter 737-200 all had the forward ones.
Mey Air had both front and rear stairs, possibly the only 737-200 charter airline to have both,
and Busy Bee just had the rear drop down door stairs (This was a gravel strip 737RC)

Charter 1-11's had both front and rear stairs. (The DC-9 30 soon followed, and that too was self sufficient for ground handling)
Caravelle's all had rear stairs.
727's had rear stairs, but some also had the forward air stairs option -- Wardair was one, but Condor and Transair Sweden's did not, nor did AFA, TIA and Worlds'.
The second-hand 727-100's for Dan Air and Hapag Lloyd all only had the rear air stairs.

The 737 front stair option continues to this day, but Charter airlines TUI, Neos, Enter Air and Smartwings have not selected them on their MAX fleet,
but Ryanair (plus Buzz and Malta Air) has opted for front stairs for their new 737 M8-200's.