quarter turn mount that screws to the underside

Nope. forums.garmin.com/.../the-mounting-plate-replaceable-tabs-broke

This part is a service part supplied by Garmin Support. Please contact your local Garmin Support Center.

Unfortunately Garmin does not have this part for sale, at least in Europe. They only offer replacing whole unit to new refubrished - under warranty or for a half price of the new unit if warranty void (broken plate by your fault).

Which country do You live? (which Garmin Support told it to You?)

Poland

i thought so that not the German/Austria/CH branch... Yep, these eastern-eu branches (like the hungarian) are totally different in pov of customer support than the german , the hungarian Garmin (Navi-Gate Ltd.) wanted to replace a 530 for ~200 eur or similar (it had display issue by manufacturing design flaw, the gluing was bad), the german support replaced it by free. these eastern companies are robbery bands, and not work for the customers...

I was told they are not able to fix anything, only can replace broken units. But this quarter turn part may be available as accessory in the future.

For now I fixed that broken plate with "Super Glue" hoping it will last for some time until any 3D printed replacemnt will be available on the market.

yes, they are not reparing, they are replacing, it is the policy of the Garmin, just as You see, there is big difference between Garmin and Garmin (support) depending where you live... (but probably the Garmin HQ doesnt know it , they are "blind" a bit, and these independent regional companies are doing what is the best for them and not what is the best for the customer... )

Thanks all for the info.  

One is replacement for the original and the other is a strong version to make sure it does not break as easy.

https://www.thingiverse.com/thing:6101504