Just email the guys at E13, you'll get a response promptly, I'm sure
I've never heard of a non-ISCG E.13 guide, but there's a new standard coming down - ISCG05 or something. Possibly you got either a brand new Sunday frame with that standard, or a brand new E13 guide with that standard?
All E-13 kits are ISCG. There is a new ISCG 05 standard which was only used on the Sunday and I believe the 7point series for 2005. The new 06 E-13 kits will ship with the ISCG 05 boomerang and an adpater to to do a BB mount.
I almost did this, bought a SRS that was ISCG 05' and found that frames with older ISCG tabs it wont bolt to. So I got my hands on a regular SRS guide. I have regular ISCG tabs though...
I think the roundness of the hole is larger, the whole ISCG deal is different. You have to use their adapter if you want to make a ISCG 05' chainguide work on a regular bike where as their older guides you could use the sandwhich method with their adapter OR use ISCG.
God that's confusing. On the DH bike I just use the sandwhich mount, on my street/BMX bike I think I'm going to use the ISCG adapter.
You'd think that E.13 (well possibly) would trade backplates if needed instead of you buying another $50 backplate. They might but I have no idea.