My understanding is that there are three basic SHAPES for acoustic flat top headstocks: (A) A "Gibson-like" shape some refer to as the "lips" or "open book" headstock, found in the 50s and early 60s; (B) the (what I consider classic) Guild "center raised" or "domed" headstock from the early/mid-60s on, as found on your D-25 (and 50, as far as I can tell from the limited view); and (C) the "snake-head" or "tapered" version of the center-raised headstock (like your D-15) found on many models from ~1987 onward (I believe this shape may have been influenced by George Gruhn). There were a few flat tops with Artist Award-style headstocks (these might have all been special order... John Denver had one or more).
From there, there are also many variants of the logos and inlay, ranging from the simple silk-screened GUILD on your D-25, to the black overlay/pearloid-inlaid Chesterfield on many mid-range models like the D-40 and D-50, to the Pearloid G-shield and GUILD logos on the D-55s. The Chesterfield itself went thru several modifications, from a three-lined column to a single piece column as well. A lot of these changes that are found up to 1977 are covered in Hans' book. The "snakehead" or "tapered" headstock is post vol I. There were even periods of variation and experimentation on these themes (some brief periods with inlaid GUILD but no Chesterfield, a period with rosewood overlays instead of black plastic, etc.
Today's "traditional" and "standard" series headstocks do differ in their overlay... with the "traditional" models having the black overlay and pearloid inlay found in previous years. However, there were no "traditional" and "standard" series prior to New Hartford. A D-50 or D-40 from Westerly is just that... a Westerly D-50 or D-40.
Hans certainly is the expert... and I would guess he will include the variants since '77 in vol II of the "Good Book."