I would check with Brownells or Coles
www.brownells.com
http://colegun.com
This is off of the Beretta USA website. Hope that it helps:
For field and sporting guns:
First the shooter should decided if any adjustment is needed. To do so the shotgun should be pattern tested at your local range to check for point of impact. Fire 3 aimed shots at a pattern board from a distance of 32 meters, using the full choke with the shells you would normally shoot. Compare the pattern impact area in relation to the point you aimed at. If the pattern is generally centered around your point of aim, no adjustment is needed. If your pattern impacts higher then your point of aim you would need to lower the comb (use a higher numbered spacer) , if the pattern impacts lower then your point of aim, raise the comb (use a lower numbered spacer). Instructions for making these adjustments is in your manual. Remember that the spacer and rear plate must match the same setting or damage to the stock may occur. The factory installed shim is the C-60-DX. Please keep in mind that these adjustments will help center your pattern but there is no way to completely fine tune the point of impact, different brands of shot shell and shot size can have a major affect on the actual pattern density and impact point.
Understanding the spacer markings:
Each spacer and plate is stamped with a letter - number sequence, for example: C-60-DX
C = Field / sporting gun
60 = The actual drop at the heel measurement in millimeters
DX = The cast that will be applied
Each spacer and plate set will give 2 different comb heights:
The stamped number corresponds to the amount of drop that will be realized at the heel of the stock, for example C-60-DX will give you 60 millimeters of drop at the heel. If you want to raise the heel higher you could install the spacer and plate to the lower number setting of C-55-DX - now the actual measurement at the heel would be 55 mm of drop. Using another higher numbered spacer like C-65-DX would lower the drop at heel by 5 mm more then the standard C-60-DX spacer.
The cast measurement is set at a value of 4 mm and can only be switched for right or left hand use. If you shoot right hand, always set the spacer on "DX" , if you shoot left hand use the "SX" position.
For trap and target shotguns:
The targeting and adjustment instructions are the same with a difference only in the shim markings: T-40-DX
T = Trap / Target
40 = The actual drop at the heel measurement in millimeters
DX = The cast that will be applied
Trap and target guns have the T-40-DX spacer factory installed. The additional spacer is a T-42.5, note that this spacer has zero cast. The "C'' stamped spacers cannot be interchanged with the "T" stamped spacers, the drop measurement would not be correct due the higher Monte Carlo comb of the trap and target stocks. The exception to this is with the 391 Teknys Gold Trap, on this model the drop dimensions are calculated using the C type spacer. The C-60 spacer is factory installed on this model and extra spacers are not supplied due to the adjustability of the comb.