...and the rear starboard service exit is the same size as the front starboard service exit, a configuration only United had.
Just check United's DC-8-61, they also had an extra rear wing emergency exit on both sides, bringing the total number of exits on each side to seven instead the regular 6.
During the late 60s, both Boeing and Donald Douglas adjusted the layout of the exits to the requirements of their customers. Donald Douglas much more with their DC-8-61/62/63, e.g. door 1R on the DC-8-61/63 can be right after the flight deck and the two lavatories and in front of the first row of seats (DL among others), or door 1R can be after the first few rows right in front of the wing (UA, KL
among others). Then the larger aft wing emergency exits could also be a full size door either closer to the overwing emergency exits like AC
, or closer to the last exits like EA
. Not to mention the layout of the windows... Another interesting aspect is that only the DC-9-31s for Eastern had a much larger starboard service door than all the other DC-9s and MD
-80s. Boeing offered different sizes of service doors on the 707/720, you will find airlines with two regular small starboard doors, some will have the front door large like AA
or Western - and the same size as on the B-727, or both being large like United on their 720s.
I remember that United wanted to introduce a high-density configuration on the B-727-222 used for intra Californian flights. But it was also the time they went for a five abreast coach seating on the DC-8s, but I do not remember whether the B-727s had also this configuration. The latter delivered B-727-222s didn't have the extra exits. During the 80s, all the extra exits on 727s and DC-8s were blocked as seen on your picture, where the exit no longer is marked.
My favourite planes flown: Lockheed 188 Electra, Tridents, VC-10, B-707, L-1011, A330, E90 + Concorde