First to remember there are Two kinds, one is Vertical and the other Horizontal.
My machine was built in 1993. Filters for that age of a machine, are less then available.
This will fit into the input of an older machine as long as you have the proper configuration Horizontal/Vertical. The trick was pressure alarms after turning on. The older machines require more airflow through the system then the newer machine. At Your Risk, you can cut a slot similar to the slot manufactured into the old original filter and that is the end of the problem.
It's not perfect, but there is only so much to an Oxy Compressor. Air in is important. Not to big (gets loud) not too small, it wont (allow pressure).
The people who sold the filter can not and did not encourage cutting holes in their filter. Rightfully so. But it worked and they gave what information they could as they are not repair people.
But it works. Its only a little louder then it had been before and I live through the night without the color blue on me. It beats buying a new one, if Medicare continues as Medicare and or living is not on the list of your insurance company. It is "Durable Medical Goods"