British flask thermos is on Tobruk. I threw one at a Panzer II, no effect at all. :s
Those weapons were notoriously ineffective againt most tanks of their time. If you place a HE bundle on a 20mm armour plate, nothing at all will happen to the tank. And 20m is what most tanks feature as top armour, allied heavies having much more. The Geballte Ladung wasn't a dedicated AT weapon, it was more like an Engineers universal demo charge, widely available. Against tanks it's only to be used when nothing else is availabe. No73 on the other hand is a real AT grenade, and it was famous for being .... useless. Things like RPG43 show how a AT grenade has to be designed to actually have some effect.
To use the Geb Ladung and No73 against tanks, you have to go for the lightest armour since it works like any other direct hit weapon. That means top or bottom hits. Of course it won't deal too much damage as it is HE only, but it can finish off a damaged tank.