Sequential part numbering as creating features & weldments

Hello, Newbie to the forums her. My Name is Russ. I've had a search but cant find anything help full yet. I've set a custom property called "Part Number". I am currently manual editing cut list properties to add this in. How can I set it up so when I create a weldments it automatically creates sequential part numbers - S1 then S2, S3 (S for sawing), And then would also like to do P1 ect for all my boss extrudes (which get profiled out). 

There might be a feature in Solidworks already, but I cant find it.

Thanks Russ

SolidworksParts And Features