I agree, it is hard to tell what the intent of the party writing this file from that field.
In general, for scans, 0 is often used. 1 was intended for Cineon style curves (based on a
printing density for film). 3 was intended for camera log formats (Viper, et.al.), However the
committee referenced in the document never came to a consensus on this subject so all that you can
say is that the file is logarithmic (with a toe or not is unknown ). It is a problem to put things in standards
that are incomplete.
There is currently a SMPTE Constrained DPX standards group working on putting HDR encodings at the end of the table (PQ,etc…)
Because you know the source, I would look at these files with an assumption that they are SLOG3.
(though SLOG2 is possible)