The Gothic book hand used by LJS 184's scribe(s) is quite typical of the late thirteenth century, and Michael Gullick confirmed in 1990 that "the script had distinctly Spanish features" (Quandt, n. 13).  There may have been more than one scribe, but (to my untrained eye) the hand looks consistent throughout.  

The rubricated letters were added after the main body of text was written, with the scribe or an editor leaving "guide letters" or "guide phrases" for the rubricator to follow.  

These beautiful decorated letters were added after the rubrication, as the ink overlap in a couple of places reveals. 

I suspect that there were at least two artists who completed the letters for LJS 184, as the "Z" changes from a yogh-shaped figure in Book IIII to a block-shaped figure in Book XXI.  

Assuming there were multiple decorators, the artist who completed the latter half of the book clearly has a sense of humor.  He has added a lovely face at the bottom of fol.  in Book XIX, "De Bello et Ludo" ("On War and Game"), apparently in an interactive "game" of his own with future readers (who seem to get it, as they too respond with doodles of faces only in Book XIX).  

The MS only has a few, relatively simple illuminations, all located on fol. 1r.  The miniature within the initial depicts Isidore of Seville lecturing to two monks, all three of whom are sitting before writing desks.  There are a few human-animal hybrids along the upper borders of the first column.  The border of the miniature within the large D and also the smaller initial letters are decorated with burnished gold over a blue base.

 

The simplicity of the manuscript's illuminations is likely evidence of its Cistercian origins. 

The large "L" (for "librum") at the top of each verso page, and the Roman numerals at the top of each recto page, were likely added after the rest of the decorative elements.  I would guess based on my New Discoveries that they were added early in the book's history, within the first 60 years of its existence, at a time when the book was rebound for the first time.  It seems likely that there were at least two individuals responsible for these figures, as the style of the "L" and overall quality changes about halfway through the text.