The main difference is the incablock shock protection. If you look back you'll see that I remarked on the serial number being very early for a shock-protected movement.
@François Pépin and
@gatorcpa are suggesting that the balance (including balance cock) is a replacement, and that the original balance would not have been shock-protected, and would have matched the color of the rest of the movement better. The serial number is on a different part of the movement and therefore matches the case.
I think their speculation is quite possible. Given that this is not a terribly collectible watch anyway, I don't think it's going to have a major impact on value. And the good news is that the watch has shock protection.
No, you're misunderstanding. Francois is saying that the case originally came with a cal 26.5, which is still there, except with a replacement balance from a later variant.