D-plate

I installed the D shim and the D steel locking plate last night, and it dropped the stock to what appears to be a perfect level for me. I don't know if anyone else has these shim and plate parts in their standard kit. I just thought it was odd they were not described in the owner's manual, or on the website. Now.... I just need to take it out shooting! I hope that someone else finds this useful.

I would like to drop the stock of my M2 tactical more. The plastic shims included with the shotgun are Z, A, B and D. No C shim is included. The D shim is not mentioned in the manual or anywhere online. Also, I have Z, A, B and D steel locking plates- no C locking plate was included. Again, there is no description of a D locking plate anywhere in the manual or online. Just by eyeballing the D plate, it looks like it will add more drop. Can anyone fill me in on the D shim and the D plate? Thanks so much?