Paul,
... the ring is marked 583 w/ St Petersburg city mark. The makers mark is under the enamel on the bottom arm of the cross, between the eagle wings. It's difficult to fully make out, but it appears to be WK (Wilhelm Keibel?). There is also a large double headed eagle mark under the enamel on the upper arm of the cross, also between the eagle wings. I've seen the double headed eagle mark on other pieces made by Keibel, but I'm not sure if this is unique.
reverse is also quite impressive...