possibly an old pressure mat hence the joints when system was upgraded ,cable seems in good condition considering the age not rough at all was normal in those days
when the alarm was done in 80's it was very unusual to gut & completely redo a house in one hit.
the cable skinned across the door way would be so it wouldn't be seen under a thinning carpet, the soldiered joints where quite nicely done
its probably given decades of service...
If you've the budget to have all the boards up & replace all the doors, why haven't you got £150 for decent controls ?