No reader for NOAA/CLASS data prior to 2000

Good morning,
SNAP is not officially made for this, but it works great for automatised processing of AVHRR images. Except that in my routine that goes from now back to 1980, suddenly in 2000, likely prior to F15, I start getting this error:
[NodeId Read] No product reader found for file …
Same issue if I try to open one of these files manually out of the loop.

Here is an example of a file that can be read: DL 38 MB
And here, one that cannot: DL 36 MB

The “extension” only indicates the receiving station - I have .WI that I can and others that I cannot read.
I’m reluctant to move to another software, and I know I am not the only one here using SNAP for non-Sentinel data, so I hope someone else has found a fix.
Thanks in advance!

Not all data is supported for AVHRR.
In the help is described which types are supported.

So probably your data is not supported.