PVC for underground

[AZsparky]

What year (approximately) was PVC for electrical underground first introduced and commonly used? Was it only schedule 40 at first, or was schedule 80 available also? TIA

 

[letgomywago]

What year (approximately) was PVC for electrical underground first introduced and commonly used? Was it only schedule 40 at first, or was schedule 80 available also? TIA

Post ww2 idk when schedules were introduced.