As far as I can see it is older default weapons - but it isn’t all of htem - eg the Arisaka carbine doesn’t have it…the Mosin 1938 carbine with a bayonet has it, but not the pre-bayonet version that I guess isn’t available any more?
Rifles that were the default weapons for new soldiers in old campaigns and are available in the research tree are marked with D (Default) when transferred from the old campaigns to the new system. Having a Default weapon does not unlock it in the research tree, unless that weapon is unlocked by a squad that the player already had. For example, if the “52nd Mixed Engineer Battalion” squad of engineers was unlocked before the update was released, this squad and the Carcano M41 rifle will be unlocked, while for example the Kar98k Kriegsmodell will not be unlocked. These weapons are fully identical to the standard upgradeable version in terms of characteristics, except when sold for Silver.
As helpful and respectful as this whole topic was, I’m more impressed that the question “What does the ‘D’ stand for?” wasn’t met with the obvious, juvenile answer. Respect where it’s due, enlisted community, good job.