I have a very similar box that contained the slate and shell stones I bought in late 1971 from Richard Bozulich of The Ishi Press. The Nihon Kiin catalogue of Showa 44 (1969) October lists the same thickness stones that you have, 2kyu, which is 5.7mm. In that catalogue stone sizes run from 37 to 25 (7.0mm) and below that are 1kyu (6.3mm), 2kyu(5.7m), 3kyu(5.1mm) and 4kyu(4.7mm).
I think you have the 2kyu stones from that Nihon Kiin catalog.
The material that the stones are boxed in is made of "standard? go equipment cardboard" - my stones are in an off-white box of that material, the cover for my table board (1971) is the same material, but dark green, and my table board 2009 from KurokiGoIshiTen has the same material but dark brown.
I'll try and add some photos.
Please note that the writing is mine - there are two types of shell.
John Tilley