"but there was one key difference to now - back then, mortgage affordability was calculated on the basis of one income, not two,"
This is not true, or at least not for all Building Societies or Banks.
Our first mortgage was in the 1970s; not only based on 2 incomes, but we had to be married, as well.
Friends of ours (a couple), the wife was obliged to sign something saying she would not give up work for children for at least 3 years; (something I thought ridiculous, as no one could guaranteee against redundancies anyway, and it was easier to get rid of employees in those days.)
(The right to return after pregnancy was only just coming in, towards the end of the 1970s)
Secondly, there was a time when building societies were restricted in their lending to only first time buyers (I think they had a limit each year), and this applied even though we had been saving with the same BS for many years. (ie, when a few years later, we had to remortgage, we were subsequently refused outright, because it was not our first mortgage).
This was a second mortgage, and so I eventually applied thorugh the same BS, but one in London, near where I worked, which had not exceeded their limit for that year.
It is generally assumed that in the past people had a single job for their entire working life. Althiugh this may apply to some people, it definitely did not apply to us. It was much easier to 'get rid' of people, and although there was nothing then described as the 'gig economy', there were, just as there is now, temporary staff, who had very few working rights.
Fixed mortgages were much rarer in those days, and it meant that repayments fluctuated with the inteerst rate, which as I still have ALL my mortgage statements, I know they reached 15.25 percent, although this was only for a short while (mainly 10, 11, 12 percent).