ok, ive had some time to look at this again. even in g3t there is no difference between x special and x accuracy aside from the index number
the mysterybyte1/2 are both 0 for all the x items (special byte too)
well in that case the effect must be hard coded to the item, and require asm to change I guess.
I ended up trying a decomp recently, and yeah the stat values for those items
are stuck in the asm, here's my recommendation,
open hex maniac advance, and go to data.item.stats table, then go to the address in the battle effect field for x attack or guard spec, you should see an anchor at the address, (a little blue "^" icon) and it should say guard spec at the bottom of the window.
Highlight everything from that icon, until you reach the next ^ icon, stop at the byte right before it. Then on the left press the code tab,
and at the top select code type Thumb.
Looking in the window it shows the first address, (ie. the offset you start at) but it also has 6 other values aligned like the offset, if you compare that to gaurd spec and the items that share its battle effect it matches up.
Guard spec and then six items. dire hit, x attack, x defend, x speed, x accuracy, & x special. (in that order)
Select everthing in the code window, and copy it all into notepad. and save that.
I believe that's the asm code for the items in question.
So look into asm and figure out how to change it, I think the last one is what you'd want to change.
Good luck.