I've got a Hitachi air con RAS-25YHA3 and when cleaning the filters the other day I noticed there are another set of small filters so took those out too.
They were filthy. According to the manual they should be replaced every year haha. Looks like the Id of them is SPX-CFH15 but for the life of me can't find any online.
Am I outta luck because it's so old?
Hitachi air conditioning products are exclusively available through Temperzone:
Temperzone National Service Centre - Spare Parts (Pickup/Despatch)
35 Bessemer Street, Blacktown NSW 2148
Tel: 1800 211 800
https://www.hitachiaircon.com/au/contact-us
If it costs too much to be worth replacing, just bin it as it's an optional add-on filter that weakens cooling capacity/speed and harbors bacteria/mold if dirty.
If you wanted to clean it , try vacuuming the filter.
Just clean them as best you can, reinstall and enjoy life.