TO, I think Mishima pottery is said to have been imported from Korea, so that's probably a good sign.
If it's not Mishima, it certainly has many qualities of it.
I have no idea what the "5-20" would be, other than a date or a manufacture lot number. The 光山 could be the name of the potter or the studio.
I searched around for some stuff, and though I found some 光山窯 (Kouzan kiln), they had nothing close to this style of item (for example, here