The snail-trap pictured below I got from my son who found it clearing out a cupboard on the Uni of Utrecht. It is out halfway the last century. The the name of the German manufacturer was on the box. But I could not find an issue on Internet. A pity. If it was for sale somewhere one would be unable to meet the demand. I quote the paper that was with it in the box

Dear aquarium fan !
With the purchase and setting up of the "wh Technlk & Hobby" trap for aquatic snails you will soon be rid of continuous trouble in your aquarium. The amount of snails is now under your control without losing much time on it as formerly when you had to pick them out of your aquarium with difficulty. You can destroy the snails you have caught. If desired you can return some species into your aquarium. So you can keep a choice stock of aquatic snails, which is very useful. It is no longer necessary to change the water because of snails, indiscriminately destroyed by chemicals. It is no longer necessary to remove the lish. Putrefying bodies would poison the water, but now there are no more dead snails to be removed. You know what it has been like up till now. Chasing snails is over.' You can leave that to your "wh Technik & Hobby" trap. The biological balance is easily disturbed if the aquarium is overstocked with fish and snails. It is well known that water plants encourage the biological balance. But they only can develope properly if they are not always disturbed in their growth end decimated by to many aquatic snails. The "wh Technlk & Hobby" trap works very easily. There are two ways to use the trap. In any case bait is necessary. We recommend fish-Feed tablets.

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This trap can be. Employed as follows:
1. Fix bait basket with bait on to base (sketch 3r at pos.1 end lay them on the bottom of the aquarium. After Borne time the snakes gather round the bait, which is hidden. by the basket. TM snails you have caught can be removed every 2-3 hours.
2. Same as described above, but fix bait ! end basket .at pos. 2j of the base. Now press the case (sketch 1) on to the base until it clicks into position.
Pay attention to the safety catches. They must point inwards at the entrance of the trap.
Now turn the base until the basket is at the point furthest away from the entrance. Before you lay the trap down on the bottom of the aquarium, Iet the bubbles escape. The entrance must be closed ,by the safety catches Snails, which are caught in this way cannot escape.
As aquatic snails are busiest in the darkness, we recommend setting the trap according to method no. 2 at night.
Please note
The snails are attracted by the aromatic substance escaping from the bait!. If the water is disturbed we recornmend calming it to enable!e the snails to locate the aromatic substance.
If soma of your fish should try to enter the trap you can move its entrance In front of a stone or aquarium-pane