Heritage Certificate

Can anyone with a heritage certificate for an X Type tell me the easiest way to find the gearbox number?  I’ve applied but been asked for “3 out 4 of the following” VIN number, Engine Number, Body number, Gearbox number. They go on to explain that the body number won’t be found as it’s “in the white” meaning not visible and I understand the gearbox number is where you won’t find it without taking the gearbox out. They require 3 out of 4 for security reasons. Well the VIN and the Engine number are on the document so we are just talking about the gearbox number, yes ? So the only cars with certificates have had their gear box removed ? Surely not. What am I missing ?