The Class 121 DMBS can be compared to a Class 117 DMBS an extra cab on the rear.
It seated 65 in two saloons in 2+3 seating, 45 seats in a saloon behind the cab, with a doored partition dividing it from a 20 seat saloon, a further door dividing it from the guard and luggage van. All seating was second class, initially the smaller saloon was designated non-smoking.
The DTS was the equivalant of an unpowered Class 117 DMS, with the same internal arrangement (in both un-ganwayed and gangwayed styles). Again it was all second class seating, the front two saloons were identical to the DMBS with 45 and 20 seats, although with no door on this dividing partition, this time the non-smoking section was in the place of the guards van in the DMBS with 26 seats, totalling 91 for the vehicle. The rear seat was full width seating six. Two vehicles were fitted with a gangway (54287 by the end of 1988 and 54289 by the end of 1989) which divided this full width seat into a pair of two-seaters, reducing capacity from 91 to 89 seats.
On the outside of the vehicles the two half windows in place of a full window was a sign of where the partition were. A four-character route-indicator box was situated on the roof dome above the middle driving cab window, and this was the flat-top type similar to the majority of Class 117s. The destination indicator was at the top of the inside of the center cab window, and was the wooden box type as fitted to most Derby vehicles.
Two marker lights were always fitted to the cabs, and there were two horizontal handrails fitted below the two outer cab windscreens and two vertical ones between the centre cab windscreen and the outer ones. A variety of buffet types were fitted, in the early days mainly the oval and 'cut' type were normal (it was not known to have one oval and one 'cut'). Latterly the large round was the standard. On the DMBS the exhaust pipes rose on the guard's van end, these spreading outwards just above cantrail level to avoid the headcode box.
The standard power train was fitted, originally AEC engines but replaced in later years with the Leyland 680.
Another external change in later days was the addition of high-intensity headlamp in-between the marker lamps. Accident damage saw the 55033 get a replacement cab roof dome with a destination box rather than a route-indicator (as in the style of a 116 / 122). Cab handrails were originally hollow, doubling as a drainpipe for the cantrail gutter, but these were switched for a solid type which no longer went to the cantrail level. Heater grilles appeared at the side of the guard's van after refurbishment.
Vehicles that still remain in passenger use now have a secondary door locking system, an extra plate extends the doors down to the stepboards where electro-magnets are fitted.