After market O-ring kit for the senco-matic model PW staplers, all the O-rings in the tool.
Also have these on ebay Senco PW Rebuild Kit
You can buy it here for $8.75 delivered with free shipping or on ebay.
The first kit below, O-rings + seal LB5002 matches the Senco kit YK0378, EXCEPT it does not include the bumper BB0101
+ Washer Firing Valve Seal LB5002
All O-rings in the tool.
Schematic break down and O-ring reference sheet with every order.