The portion before the slash is the actual WAL file itself that it got to start replaying, usually the last few characters of the WAL file name. The portion after the slash is the position in the WAL file itself. Default size of each WAL is 16MB, so if that first portion doesn't match the primary, you can count back that many WAL files x16MB. If it's on the same WAL file, I'm not really sure how to get a more precise byte lag than that.
Keith