I have two philips TUV T5 germicidal lamps, one is slimline single pin and the other is 4 pin of one side (other side is blank). I just noticed what could be a standard T5 metal endcap inside the plastic, which makes me believe these are just standard T5 endcap germicidal tubes with a plastic cap stuck on, which converts the 2-pin endcap to either single pin or 4-pin.

Here are some drawings of what I think is inside these lamps:

I am pretty sure this is exactly whats in there

The lamp may still be glued in, or the wires might be pulled out (stretched) through the external pins after the plastic cap assembly, preventing taking the tube out

So these lamps are the same internally. The thing is the "special" lamps with plastic endcaps are being sold for $15 more than standard variants!

How else are all the fancy smart water bar companies going to make money ?

True lol. I saw an ad for one, which had a very fancy socket- 3 pins on one side and a single pin on another. Turns out, the brand of the filter takes regular philips germicidal lamps and puts their own custom base, so normal lamps can't be used. The brand is the only source of those tubes...

Back in the mid 00s most water bars sold here were the generic types, with mechanical water valves, most parts interchangeable. No filter and no UV lamp. You installed any standard filter on the input water pipe

From ~2020 onwards, most water bars are the smart types with touch buttons, custom filter, custom UVC lamp, and the firmware to detect when you last replaced them and nag accordingly

Here is also an germicidal lamp with unusual long bases under Heissner WZF405-00 name. T5, 5W, lenght 18cm: https://www.tuontiukkonen.fi/tuote/heissner-uvc-lamppu-5-w/

This is much more elaborate. I wonder if the glass tube is same in length as 4W

Yes, it most likely has a normal 4 watt glass tube. Naturally, the price is quite high...

But this luminaire or appliance could be easy to hack with an ordinary G4T5 + detaching one of the sockets from its place and connecting it on the lamp pins

I saw a picture of this lamp a while back:

https://www.lighting-gallery.net/gallery/displayimage.php?album=search&cat=0&pos=1&pid=205560

But here it looks like they use the long caps to make a short lamp into the standard lamp length ?

It is not clear what kind of device the lamp is intended for. Maybe there is also a more powerful version of the device?

Interesting. Couldn't find a decent picture of what the Heissner lamp looks like in reality. https://www.ebay.de/itm/146628170200?var=445316026296&mkevt=1&mkcid=1&mkrid=707-53477-19255-0&campid=5338722076&customid=&toolid=10050
Here is also this other version with a G10g base, as mentioned at first post.
https://www.heissner.de/uvc-ersatzlampe-5-watt-sockel-t5-g10q-austauschlampe/WZF405LV-00