... are now the standard "default" buckle on new Lange watches that are (a) priced above a certain level and (b) delivered to dealers/boutiques only in the US market. So what is that price point? I was told that deployants now come standard in the US on Langes that are generally priced above the "basic"
(uncomplicated) Saxonia and 1815 models. Less costly Saxonia
and 1815 models will continue to come with the tang buckle.
My understanding is that watches that were delivered to US dealers
before Lange made this change will not be retrofitted with deployants.
Those older watches that have been sitting in inventory will be sold
with their original tang buckles.
So exactly what type of deployant would come with your watch? Most of these new Lange models come with the (newer, thinner) double-fold "butterfly" deployants shown below. However, I was told that certain models, including the Datograph Up/Down, the Double Split, and the Richard Lange Pour le Merite, now come standard with the single-fold (older, thicker) Lange deployant. Again, this is for the US market. I'm not sure if this happens in other markets.
Regarding your question about interchangeability: Lange 1 MP buckles are 16mm. Dato buckles are 18mm. Therefore, they are not interchangeable with standard straps. But of course, you could always use a different strap (e.g., put a strap on the Lange 1 MP that uses an 18mm buckle rather than the standard 16mm buckle). You can look up buckle widths yourself by clicking the "HISTORIC DATA" tab on this spreadsheet: http://goo.gl/vWFC7Q.