This has been prompted by another post when I thought I could assist but not!
SECO inserts type TNUX 220408 also marked P30 and S4, I have 44 of the things but no holders. They are triangular shaped, convex on both sides.
I may be wrong in thinking they do not match the other description but if proved wrong and of use let me know.
Or anybody else has a definate use for them let me know.
Adrian