From prior experience helping with the deacquisition of about 140 tsuba from a small museum, the first two numbers (28) usually reference the year it was taken in by the museum. Then pull the old books kept by the museum and look up acquisition item 722 from that year.
The collection I helped with was largely taken in 3 waves during the 1920s and 1930s. I still have two of the tsuba that I bought from it.
Over the years, I have had other tsuba with Museum #s.
Currently the only one I have is this big thin Ko-shoami or Ko-Kat.
For various reasons, I know what country it came from but not what museum.
Anyone have anything similar and information to share, do let me know.