I have some 2179's, too.
And according to Omega, there were 10,000 units delivered to US Army at that time and they have /3 or /6 or ??? following 2179.
Today, I took two of them out and compared.
they are 2179/6 and 2179/3
Strange enough, the backs are not interchangeable.
In this picture, the back is from 2179/6 the case in 2179/3.
/6 on your left, /3 right
I looked closely at the case, they are a bit different, too.

Funny .