Needs a picture to be certain. But it sounds like a series street lighting lamp.
20 A probably refers to the current rating, 20 amps was the largest common current rating.
6000 l Probably refers to the light output, 6,000 lumens.
Large numbers of lamps were burnt in series from a high voltage constant current source, these lamps could be of differing sizes provided that all had the same current rating.
These lamps were rated by current and by light output, and not by wattage or voltage.
Very little used these days, but AFAIK at least one installation still exists.
A somewhat similar system is used today for lighting airfields.
If you wish to try the lamp, you need a very low voltage supply that can supply 20 amps. A 6 volt vehicle battery would serve.
