Re: PA28-161 POH Revision
Flight Manuals are normally only available from the manufacturer and are issued for a particular serial number aircraft.
As Ghengis says, someone is playing the game if they let you fly the aircraft and don't grant you access to the flight manual - it must exist as it forms part of the CofA.
The best thing to do is ask the flying club if you can borrow the FM one rainy day and the aircraft is on the ground and photocopy it. Remember that aeroplanes differ so the FM must be the right one for that particular aircraft.
As I mentioned earlier, later PA28-161s have the battery on the firewall (rather than under the back seat) and a MAUW of 2440lbs (others are 2320ish lbs IIRC). The lightweight ones can be modded so you have to know what's what for a particular machine.