I have a Ruko in .177; it is basically a Diana Model 28 with different badging. The trail for Ruko air rifles goes pretty cold after 1995. Apparently the company had dealings in the Phillippines up to 1995, but after that, who knows. If you can find markings on the left hand side of the receiver that indicate the gun was made in Germany, it might be a Diana, meaning replacement parts should not be a problem to source. If not German, it could be anything. Do you have any detailed pictures of the mystery gun?