Bulova 1960 Countess

This is a 1960 ladies Bulova. The case is 10k white rgp. It has a 5AR 23 jewel movement with a "M4" date code that is difficult to see. Apparently the movement has been changed at some point. I think the 5AR movement is probably the correct movement, but just the wrong date code. This watch is very tiny. Just got it yesterday and it's kept perfect time for about 30 hours now. I think it's a 1960 Bulova Countess "B". There's not another one on here to compare it with. See what you guys think it is.