How to tell which 6139 movement you have A or B?

[SunTzu]

How can you tell if you have a vintage Seiko 6139A or 6139B movement?

Do I tell by the serial on the case back which dates the Seiko? Because the 6139A was used until 1971 and 6139B was used after. That's what I've read. Just want to make sure that's correct.

I have a "Pogue" 6139-6002 and "Bruce Lee" 6139-6012.

I sourced gaskets and crystals for them but I hear the ball bearing hitting when I give a quick flick of the watch in my hand. I want to source new ones and they're inexpensive but need to make sure I get for the right movement.

Thanks.

[GEORGE505]

If you can open the watch look at the movement numbers. It should tell you a,b,c after the movement caliber. Should be stamped on the main plate of the movement

[SunTzu]

Thanks. Thought I could figure it out without opening it Don't have the tool. Should get one.