Found the number 8970 as the MPC GG1 model they were modifying. An e-mail to Lionel generated the following part numbers for the washer and spring.
Spring 6308064049
Washer 6308302P15
The service tech recommends using 2 washers over the hook which slips into the frame, with the spring sandwiched between the washers. I know they have these in stock. I will order enough for 2 trucks, install, and test the operation. Will post my findings.