Nailer ID pictures, installation Video and schematic download links below

LB3500 is the seal in the valve BA0103 at the top / back of the gun, with age they get brittle and break

Senco LB3500
Senco LB3500
IN STOCK Ships same Day or the Next
Senco Seal LB3500 installs on valve BA0103
For Nailer SNIII SN3 SN4 SNIV SN70
Ships Free US
Available Qty: 9

This is the above seal + complete O-ring kit and the top piston bumper BB0059
BB0059 stops the bolt in the center of the driver from damaging the upper valve assembly.

Senco LB3500 + SN4/SN70 O-rings + BB0059
Senco LB3500 + SN4/SN70 O-rings + BB0059
Senco Seal LB3500 + O-ring Kit IN STOCK
For Nailer SN4 SNIV SN70
Ships Free US
Available Qty: 10

This is all of the above, + The lower bumper BB0018

SN4 SN70 O-ring kit + LB3500 + Bumper BB0018 + BB0059
SN4 SN70 O-ring kit + LB3500 + Bumper BB0018 + BB0059
SN4 & SN70 O-ring kit + Washer Firing Valve Seal LB3500 + Bumper BB0018 + BB0059
Price includes shipping

This is the driver kit, Plus LB3500, BB0059 and driver O-ring LB0132

Senco SNIV SN70 Driver Kit YK0027
Senco SNIV SN70 Driver Kit YK0027
OEM C Kit
# YK0027 + LB3500 + BB0059 + LB0132
For Models SNIV and SN70
Available Qty: 2

LB3500 with or without the complete O-ring kit, the SN4 and SN70 share all the same O-rings.
If you have the AST 4 stapler, let me know, i can supply a kit for it also.

sn4-sn70
Download the schematic
SN4 SNIV
SN70