I seem to recall that in British Rail days, particularly before fixed formation sets became the norm (so probably until about the mid to late 1980s) trains on the West Coast Main Line, especially Euston to Liverpool, Manchester and Glasgow trains, were often formed of a mixture of Mark 2s and Mark 3s in the same train. I think the Birminghams, Wolverhamptons, Shrewsburys and Holyheads were pretty much always Mark 2s apart from the buffet car and brake car, though (and so were the Blackpools until their last few years of operation).
Where a train was formed of a mixture of Mark 2s and Mark 3s, was there a set number of Mark 2s and Mark 3s that would normally be found on each train, or were they just coupled together at random? If the latter was the case, or if a Mark 2 was ever used in place of a Mark 3 (or a First Class in place of a Standard/Second Class coach, which did occasionally happen) that could potentially have caused problems with the seat reservation system.
That said, except at busy times or on trains with compulsory reservation (or shown in the timetable as reservation strongly recommended) passengers who reserved a seat were very much the exception rather than the rule in those days as there were much fewer people travelling than there are today (or at least than there were before Covid19). Also, those were the days before the internet so you had to go to a station or BR appointed travel agent to make a reservation and it was more of a faff than it is today, and there were no Advance fares (at least not as we know them today, although in the 1970s BR did offer an Advance ticket of sorts called an Economy Return that you had to book a week before the day of outward travel) so long-distance rail travel was primarily turn up and go unlike today when it is primarily book in advance.
Because of that, I would guess that in those days if someone's reserved seat in the booked rolling stock was missing it was pretty easy to simply allocate them a different seat.
There is also this now-closed thread on WCML Mark 2 formations at www.railforums.co.uk/threads/west-coast-mk2-formations.69019/
Where a train was formed of a mixture of Mark 2s and Mark 3s, was there a set number of Mark 2s and Mark 3s that would normally be found on each train, or were they just coupled together at random? If the latter was the case, or if a Mark 2 was ever used in place of a Mark 3 (or a First Class in place of a Standard/Second Class coach, which did occasionally happen) that could potentially have caused problems with the seat reservation system.
That said, except at busy times or on trains with compulsory reservation (or shown in the timetable as reservation strongly recommended) passengers who reserved a seat were very much the exception rather than the rule in those days as there were much fewer people travelling than there are today (or at least than there were before Covid19). Also, those were the days before the internet so you had to go to a station or BR appointed travel agent to make a reservation and it was more of a faff than it is today, and there were no Advance fares (at least not as we know them today, although in the 1970s BR did offer an Advance ticket of sorts called an Economy Return that you had to book a week before the day of outward travel) so long-distance rail travel was primarily turn up and go unlike today when it is primarily book in advance.
Because of that, I would guess that in those days if someone's reserved seat in the booked rolling stock was missing it was pretty easy to simply allocate them a different seat.
There is also this now-closed thread on WCML Mark 2 formations at www.railforums.co.uk/threads/west-coast-mk2-formations.69019/