Ah well, there's Isofix and there's Isofix!
Very early systems had the metal hoops between the backrest and cushion. The kiddie seat has hooks that engage with them so they're a set distance apart to match up with an Isofix seat. This might be what you've found.
Later, it was adopted as an official part of ECE Regulation 14 (part of the one for Seat Belt Anchorages) and proper tests were developed. During these it was found that some forward-facing kiddie seats were likely to nose-dive in a big crash so manufacturers started adopting either props that support the front edge of the kidde seat off the car's floor or "top tethers" that hold the top of the back of the kiddie seat to the top of the car's seat beckrest (or go right over it and back down to the floor behind the car seat backrest. At about the same time, they also adopted an internationally accepted system for labelling the anchorage points was developed - a pictogram of an anchor for the top tethers and little plastic buttons with the "Isofix" word on them near the hoops behind the seat cushion. The handbook should tell you what sort you've got but I guess that IF they are anything to do with Isofix AT ALL, they will be the early version.