A number was often used on such locks (and firearms and other mechanical products) to ensure that nobody tried to mix parts from one lock to another. The number is sometimes the last few digits of a serial number, or simply cycled from 1 to 99 for example as items were produced. Thus, if a locksmith had several similar locks on his bench it would be unlikely to get parts mixed up. This was important if the parts had to be hand-fitted and thus not interchangeable. It’s not much of a problem today but I believe might still be seen on firearms.
So your 78 carries no real significance, except collectors like to see the same number on all components rather than a mix.
