As of now there is no way of buying bayonets for your guns if they are missing them, you have to buy new weapons to get them which is very expensive in some cases.
So I suggest adding the ability to buy bayonets of weapons you own, including event, premium and BP guns. Since you will be able to freely buy them the ability to use them as combat knives instead of the regular ones should be added with the exception of spike bayonets.
A visual display of how the quick buy menu would look:
Technically, it would be like with the mines, where you can choose from different available models, right?, the same could be considered for grenades for example we haven’t seen any German M39 grenades, even though they were more widely produced than the iconic M24 or its improved version, the M43.
I think it would be better if we could buy and attach bayonets to weapons that don’t have them yet, such as the Lanchester and Lanchester 50. The Japanese SIG 1920 completely outperforms them and even comes with a bayonet by default.
The FG42 and many other guns already have bayonet fittings. Only Japanese machine guns are rather special in this regard.
Which is why I suggested the ability to use some of them as melee weapons, that way the bayonet shop could get more use. It would be neat to use the cool bayonets instead of the default knife.