there are standards:
ISCG OLD
and
ISCG 2005
the '05 arrived because the royalty-free standard of ISCG (not originally called "OLD" but just ISCG) was misunderstood by designers and frame fabricators and therefore frames arrived with the ISCG mounting holes "rotated" to suit the width or height of particular chainstay / swingarm tubes
ISCG 05 was designed to improve this by increasing the BCD (bolt circle diameter) and also indicating the mounting holes relative to the frame geometry to ensure that a chain device could be bolted straight on
HOWEVER....certain frames which use chunkier tubesets will still not accept certain ISCG chain devices as the thicker chainstay or swingarm will not allow the mechanic or home builder to rotate the backplate to line up with the mounting holes
for example, the Mythic Scythe will accept an E13 SRS ISCG 05, but not the E13 STS ISCG 05, because the STS takes up to 36T chainrings and the Scythe's thicker swingarm does not allow the smaller STS to rotate sufficiently to line up with the mounting holes - this is also true of other brands
with a frame like a Specialized SX Trail the ISCG OLD mounts are actually rotated forward to clear the low slung swingarm, and an E13 SRS ISCG OLD will not fit unless raped with a file and hacksaw....
E13 are actually pretty good in providing a "fitment" chart to help people get the right device, for example the old banshee scream frame required a "wide" backplate rather than the std. E13 backplate
this is why its good to have an experienced. bike mech. fit your chain device, the best £20-30 you will ever spend
