General rule, which the factory regularly broke - If the serial number is on the butt, the gun shipped with standard hard rubber stocks. If the number is on the frontstrap, it shipped with targets. If, as Ralph mentioned, the absence of a number on the latch suggests a fixed sight gun was pulled and modified, the factory would not relocate the existing number on the butt.
It may have been a matter of record keeping, but not mentioning target stocks usually meant standard. Bottom line, since not specified in the letter, a case can be made for whatever you have handy.

Bob