Need air filter size for older York Heat/Air unit. Circa 1995.

I have an older York central heat/air unit that has an oddball filter size it seems.
The sticker on the filter frame reads as follows:

FILTER FRAME ASSEMBLY IFR0604A

Central Environmental Systems
631 South Richland Avenue, York, PA 17403, USA
035-04715004 REV. C

Does anyone still have this old information or happen to know for certain.
It was installed in February, 1995.
Google shows nothing, nada, no way, no how.

It appears that it is probably either 21 x 26 x 1 or 21.5 x 26 x 1

I'm leaning towards 21.5 x 26 x 1, but I have to order them online and I would
prefer to get the correct one as they are expensive.

Any help would be greatly appreciated.

Thanks.
James

source https://hvac-talk.com/vbb/showthread.php?2224191-Need-air-filter-size-for-older-York-Heat-Air-unit-Circa-1995&goto=newpost