You can't have BOTH the pistol grip and a magazine with the capability to accept more than 5 rounds on a postban gun. I currently have a postban w/pistol grip, but will be swapping it out in favor of a straight stock so I can get the extra rounds. I believe the pre-ban was only 7+1, which makes sense because the mag tube extension for the postban is +4 rounds (the mag tube itself really holds 3 rounds, but comes with a +2 extension = 5). It depends on what size shells you use too - I can only fit 4 3" shells in mine.
As to how to tell a preban from a postban... The best way would be to call up HK USA and get the serial number cutoff. HK imported all the Benelli's until sometime in 96. But, there are some things you can check quickly as well. All prebans will be marked HK (since they were the importer), but since HK imported them until 96, not all HK marked guns are preban. The prebans also had one piece mag tube, but I believe there were some postbans with one piece mag tubes w/o the pistol grip sold as well. Sooo... if you find someone selling a Benelli as a pre-ban, you can be sure they are lying if it isn't marked HK and if it has a two piece mag tube.
It is also interesting to note that on all of the pre-bans, and some of the postbans (I believe they changed it around 98), there is a trick where you can actually insert an extra shell in the carrier, so you can have a pistol grip, 5 in the tube, one in the chamber, one in the carrier = 7. The story is they changed it because hunters were using this to get their shotguns to hold 4 rounds, but I'd bet the real reason was the ATF and a magazine that could essentially hold 6 rounds...
Rocko