RDS Singlespeed Drop Outs

Has anyone tried fitting Magura IS calipers to the RDS singlespeed drop-out......well they don't fit. The front leg fouls the inside fixing bolt. V annoying !! I don't want to get grizzly and grind a 4mm section off of the drop out after spending £80 on the things !!!

However a postmount caliper with Post-IS adaptor fits, but you still have to grind a section of the adaptor off, but at £5 a pop I didn't really mind!?

Anyone had this trouble?

Ended up swapping brakes over, as the price for one postmount caliper was silly !!