Low mtce battery

Just fitted a new DIN53L MF battery to my 1987/88 model. On one end, but not the other, near the top and on the side is a very small red plug (say 2mm diam). What is that for and should it be removed or left in?

A picture would help

Vent hose attachment if battery is installed in trunk or enclosed space? Under hood, not necessary so plugged.

Here we are. I would stress that this plug/screw thing is extremely small (hole bore would be about only 2mm and the plug/screw is only to one side.

Just to alert any followers that I have now (as an edit) placed an image on my above post, as requested.