Even in the 50s, 40s, 30s, 20s, etc. all the way back to the dawn of time - there have been extremely potent varieties (strains) of cannabis.

I would say many of the exotic and "pretty" species were lost due to governments ploy to strangle any sense of enlightenment - but some are preserved by the smart grower.

All in all, potency is always relative to the bud you are smoking - so in essence, potency has not changed; the amount of buds that ARE potent has increased. It was always there, just not so signficantly to the average toker.

My2c