Dated Beer Bottles

I spend a reasonable proportion of time looking for birds and plants in the Nature Reserves and Travelling Stock Reserves (TSRs)  in the Canberra Region. 

Knowing how thirsty I used to get working with livestock it is not surprising that from time to time I come across the occasional beer bottle in these places.  What is surprising is that many - possibly most - of them date back to the 1950 and 1960s, when the bottle manufacturers used to 'cast' the year of manufacture in the base of the bottle.  Note that I'm using the term 'beer bottle' to refer to an honest 26 fluid ounce container not a stubbie.

The question is why is there this strange distribution of ages?

My original suggestions were:

• People stopped drinking in the 1970s
• Drinkers stopped going to TSRs in the 1970s

On reflection I would add to that:

• changes in farming practices such that  TSRs stopped being used by drovers in the 1970s.  (This is an important point because it might add fuel to the cause of the Government selling off the TSRs, which are often the only decent natural habitat in an area).
  

An early recipient of these thoughts also suggested (thank you Muriel):

• the 1970s were the time drinkers switched from bottles to cans.

Any comments on these ideas or other thoughts would be welcome.