In addition to the variants of the simple lined hard rubber buttplates, we know this buttplate was used by Hunter Arms Co. c. 1890 - 1910
and this c. 1886-1892
and this buttplate on Hunter Arms Fulton guns
It is possible that the subject buttplate was installed
at Hunter Arms', and at the other maker's, factories. I don't think we can know conclusively unless Dr Jim comes across the buttplate specified in a shipping record.
In that they were also used by Fox, Ithaca and Lefever, they were not proprietary to Hunter Arms Co., and as said, I don't believe anyone knows the maker.