Dan, if I am reading your post right you want to know the shell holder# for the 338 Lapua Mag in the RCBS line? If I have read it correctly the shell holder you need is either 37 or 14. I reload Lapua brass and I could not get number 37 to work - I had both 1x fired brass and 200 brand new brass - 37 is the official one from RCBS. When I contacted them they sent me two shell holders another number 37 and 14. The second 37 didn't work but 14 did. I would recommend if you have to pick one up you try the 14 first - it is very tight on my brass. Hope this helps