Is there anyone who has an idea which coin this is? [gelöst]

The weight is about 1 gram (alu) and it is 18 X 18 MM. I think it is from Ceylon.KM or Y nr Please. 

How have you searched for it already? If you use some obvious things like date, value and shape etc you should find it quite easily.

Query

I also found this, but my coin is from 1975 and is 18 X 18 MM. And not from 1978 or later and no 21.52 X 21.52MM. That's the problem, or is the catalogue not complete ??

Ehm, your picture clearly reads 1978 and the size isn't in a A x B format so it's most likely the largest diameter (probably should be changed to an A x B format that exists now for coins though).

Your coin is N#1599  without much doubt.

The image on your coin shows a date of 1978.

The coin listing probably has a diagonal size. Numista now allows length and width for rectangular coins but that wasn't always the case, and “largest diameter” is a traditional method for sizing non-round coins.

I measured my coins as 18x18 and submitted size change requests for the three coin listing for Sri Lanka with this shape.