FTA-C-76 Locking Diesel Fuel Cap Safety chain and crossbar included. Designed for use only on vehicles equipped with properly vented fuel systems.
Non-Vented Cap, 3.0"-8 UNNS Threads. Measures 2-7/8" across threads.
This is the cap used on Mack and Volvos with the Low Sulfur PNG Style Tank
mfg # 90-04776-01
Checking your original non-locking cap you will see one of these numbers on the inside if it is a PNG tank: 21118646, 16MF333M or 90-03948-01.
For Mack and Volvo trucks with 3-3/8" fill neck see FTA-C-69