We will have to wait for Webster to stop by (life is hitting him rather hard right now), but is seems to me that he has the A-Scope and PPI active at different times. So if you are in 1942, you might only have the A-Scope. Look in Webster's documentation and see if he mentions it, or look in the UpgradePacks.upc or the sub's upc files (NSS_SubName.upc and UnitParts#SubName.upc) for "clues" as to the active dates.
