They weren't happy about it - especially when they saw I only paid AU$130 each for four of the 78Sc (also got a couple of Montana 610 for AU$205) - bargain - price error
Turns out they are the g3 maps and Garmin sells the maps alone for AU$240.

I've got a GPSMap 8610XSV + 7410XSV + panoptix livescope in my boat and the extra .img appear to be routing for auto pilot.