My guess is that it's custom made. Other than the water problem and getting replacement filters I'd say it would have to have some sort of support bracket added to it or the carbs are gonna be poping out of the manifolds.
They used to be routinely available. Somewhat better than pod filters. The ideal is decent, but as mentioned, you can get water into the element.
As to the support bracket idea - this is very much needed - even with pod filters. On the few bike I've had with pod filters, I run a couple of zip-ties from the frame to the carbs to support them. A set of 4 carbs with filters weighs well over 10lbs - and this bounces up and down as you ride. If you don't support the carbs, you'll kill the carb holders quickly. The carbs shouldn't ever come out of the carb holders - there is a raised rib on the inside of the carb holders that locates into a groove on the spigot of the carbs.
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