The famous French museum "Louvre" is also famous for using heavily modified old 3DS XL units as audio- (and 3D photo-) guides (although the picture on that website shows a small O3DS...).
The multimedia application - the guide itself - is also available (in single-language versions, unlike the museum version) to the public on the eShop, as well as in the museum's shop (holding the questionable awards of being the least-retail-availability game card intended for sale to the public, as well as the only truly region-free game card!)
The console itself, instead, is allegedly very heavily modified from a retail one, in both hardware and software:
By Paper et al on www.gbarl.it on 2015-1-30 (topic deleted for no clear reason) - Pictures
It does not have working speakers (speculation is that they have been disconnected or outright missing - although standby has been reported as working, as earphones are included in the 5 € rental).
It's been speculated it may lack infrared and/or WiFi (the latter claim is incorrect, according to the few pictures of both reporters).
The wifi enabling button is fake or at least glued in place.
It includes a minimal OS requiring a (specific?) "game"card to do anything visible, in which case it appears to just autoboot it
The console contained a CTR Guide Boot Card (LNA-CTR-AAAP-EUR), which probably acts as just a dongle (it can be removed safely "10 seconds after loading completes"), and an SD card (Panasonic, 32 GB, with a cut-off sticker and another yellow-circle one on top of it) containing configuration files, logs, and software.
Inserting the card in another 3DS resulted in 173968 megabits free being displayed, and no titles installed (although Paper was probably unaware of the fact consoles with different movable.seds will look for titles in different folders).
The software on the gamecard attempts to connect to a CGBL_WAP000 network (whose password may be 0123456789012), and tries to communicate with a local webserver. It also runs the software from the SD (whether or not the network was found - at least for a limited amount of time or attempts)
By Silver_Johnes on gbx.ru on 2017-10-13
Allegedly bricked (but Silver_Jones does not appear to know about the boot card)
Contents of SD root: https://yadi.sk/i/IWFABPV33Njg6K
Content of config.txt:
Content of CGBL_VER.txt: