"MAKING SUMMER BEER-DRINKING SAFE.
A novel health edict has recently been issued in Germany. The Bavarian Ministry of Public Health has announced that open beer tankards (steins) are not hygienic for use during the summer in open air, dust-swept beer-gardens, and that they should be exchanged for containers with solid tin-covers. This appeal, which has been made to stimulate the long-established tin-manufacturing industry in Bavaria, is meeting with a ready response."
Brewers' Journal 1934, page 340.
I assumed the lids were there to stop wasps of shit falling off trees getting into your beer. Seems I was wrong.