Hi Tulio and V.C. Well, it looks like we´ll go as far as our educated guesses can take us... Here a couple more elements:
Rocket racks FG-1D/F4U-1D until F4U-4: Mk5 Mod 1 (until Mk3?)
Practice rocket launcher for these: Aircraft launcher Mk6 (this is the type of rail that is clearly seen in FAS FG-1Ds in several pictures).

Rocket racks F4U-1B and all F4U-5 series: Mk9 (Mod3) This is quite different, it is a single rack, not double like the Mk5, BUT, looks like it can take the same Aircraft launcher Mk6 for practice rockets. This type of rack does not use this adapter to carry heavier 100 or 250pd bombs.

So, I am pretty sure that salvadoran FG-1Ds had the Mk5 rocket racks, with the Mk6 Aircraft launcher for practice rockets... what I still can´t find out, is if these Mk6 Aircraft launchers had the capability to carry bombs...(

)..
Tulio, do you have by chance a picture or drawing of the Mk55 Mod 0 bomb rack? I have the impression that these were specific for the Mk9 rocket racks, they look like caps fitting over the single Mk9 rocket racks (?).
I think that, similar to what V.C. says, FAS 204 picture shows twin practice rocket tubes (Probably AERO-1A 2.75" FFAR Training Launcher) fixed to the outermost racks, and three 100pd bombs hanging from the other racks. Quite different ordnance, isn´t it?
Marco