I am surprised that given the complixity with insert and tool holders, no retailer has created a calculator where you put in the code from your tool holder and it shows you the range of inserts they offer suitable for your tool.
If you type in the toolholder reference number in the search box on the Cutwel website it says which inserts the holder is designed for – though in this case it doesn’t recognise the toolholder reference on the OP’s holder (presumably because Cutwel don’t sell this holder).
MCGNR is a bit of an odd holder, in that the only reference Google comes up with is an AliExpress listing which says the holder is intended for a CNMG 120404 (or 120408) insert.
Nigel B.