Well, under the Old Queen it was 100 hours per month and then you could have a quick medical and go on to do another 20 hours if you passed.
When I flew with a Part 121 carrier in the 1980s it was 100 hours per 30 days on domestic flights and 120 hours on international routes with the proviso that you couldn't do more than 300 hours in a 90 day period.
I think it was only with the advent of JAR Ops that the new 900 hours per year came in.