Hans, I don't remember these TRCs from the book. Are you aware of any parameters for their use? We haven't seen all of Dennis' guitar, but, aside from the color (and the nut), it looks more-or-less standard for a Starfire VI of the period.
'Custom' truss rod covers were used on 'custom order' guitars, but not consistently. I've only seen them on instruments from Hoboken, but that doesn't mean they were not used in later years. I simply have not seen a later one with such a cover.
Similar truss rod covers were also made in house for instruments of which only a small number were (expected to be) made. Early Duane Eddy models had similar oversized truss rod covers that were engraved on the front ( see page 77 of 'The Guild Guitar Book') and so had the prototype for the George Barnes AcoustiLectric ( see page 81 of 'The Guild Guitar Book'). When production of the Duane Eddy model increased, a special cover with Duane's signature became standard for the model. The Bert Weedon model, which was also produced in very limited numbers, had a similar cover, with the name applied in gold from the back side and painted over with black, in the same way as the pickguards from that particular period (see page 92 and 97 of 'The Guild Guitar Book').
As I already mentioned, not all 'Custom' order guitars did get the 'Custom' truss rod cover and this Starfire VI would be somewhat unusual in having the cover just for the 'custom order' Brown finish, but it's not the first example of Guild being somewhat inconsistent in the use of its ornamentation and trimmings.
It is also very well possible that the customer ordered that feature.
Sincerely,
Hans Moust
www.guitarsgalore.nl