@Ballebar
Thanks for your answer. As usually informative, thoughtful and nicely crafted answer. I resisted responding to it last night (it's almost 8am here now) as i felt it deserved a much more considered answer than my somewhat flippant one above. A lot of that info i was aware of, ie the file naming protocol and paths for some other nuvis like 7x0/7x5, which makes sense for the legacy units with such original naming constraints and is therefore quite understandable. But suppmap1 for gmapsupp.img in the Nuvi's Map folder makes absolutely no sense and apparently cannot be explained away as "some over-looked hangover from some long-ago software predecessor of the 1xxx line?" (my musing) or "Maybe the nüvi 1300 uses some default legacy garbage in the firmware that is just empty placeholders from the past?" (your similar conjecture). So what was Garmin up to? Was it just neccessary for them to leave suppmap1 there because they constructed the 1xxx firmware from some other existing fw as a base? Therefore it was easier/better to have it linked to 'something' than dead? I suppose that makes sense, kind of. But why to to gmapsupp.img in the internal Map folder, when at least to gmapsupp.img in .system or even to gmapsup2.img in sd/Garmin may serve some purpose. I suppose it was just easier because of some fw constraints not understandable by a layman like me. But i'm sure you could figure it out if you could be bothered, however it's simply not worth the effort so please don't try on my account.

I was also only vaguely aware of the use of Dlr DB/gmapoem.img for oem branded images and that as such it cannot be deselected. I'm also of the (perhaps incorrect) impression that Garmin sometimes uses gmapoem.img for preloaded 'normal' maps, and that they also are not deselectable and don't show in Maps/Info, is that so? However, that's not what i was on about with my original question, which was on using 'foreign' maps not normally intended for oem use. It has been answered by ddabcd277 probably as to do with the NT/nonNT type and/or some info in the header which makes sense to me. That explanation of header info would also explain way some are not even recognised as gmapoem.img at all as i had heard.

My >4GB file size comment was a bit of a 'throwaway line' really. I was also vaguely aware of the map tile number/map family number limit constraints. I'm quite aware of the [Only registered and activated users can see links. ], but honestly thought it was 4 gigabytes (GB) less 1 byte. I notice you used 4 GiB (gibibyte). A typo? No, you don't do typos.. so it really is a 4 gibibytes less 1 byte limit?