ARX is a compressed archive utility for DOS, and associated file format. It was developed by Michael Kosior, and released as shareware.
ARX appears to be derived from LHarc circa v1.13. Its header format is not quite compatible with LHarc/LHA format. Its compressed data format appears to be the same. It's unclear if it's supposed to have any benefits (e.g. speed) over LHarc.
ARX probably can be reliably distinguished from LHA, but more research is needed.
-lh1-" appears at offset 2. (One might expect to see "lh0" instead of "lh1" if the first member file was not compressible, but ARX v1.0 seems to always use "lh1", possibly due to a bug.)
The byte at offset 21 has value
0x00. This normally won't happen with LHA header level 0 or 1, because that's the filename length field.