It's possible that the rating was a guess based on the rating on the back so would be the maximum consumption 40W
All those bulbs probably took 10W
Also at this time people were advertising the output as Peek program and all sort of other ways of biging up the published output.
Heralds calculations are excellent way of finding the true output.
RMS was a way of describing the "real" output, but is also not correct but it was sort of useful for making like for like comparisons.
Cheers
Mike T