I have worked through to the part where you obtain the seal for the forged letter. I have successfully entered the Imperial Offices (using both doors at different times) opened the chest/truck to the left of legion commander's desk and while the chest contains many things, the seal is NOT one of them. The green arrow shows the desk but no seal. I have successfully opened the chest on four entirely different times and routes to the chest and still no seal. Can anyone suggest a fix? Thank you.

It's on the desk, not in any chests, just activate a blackish object on the desk, it'll seal your letter automatically.