The blocks are accessories schedules.
http://en.wikipedia.org/wiki/SOPMODThe mods are the difference between the drop in RIS and the free floating RIS with a low profile gas block.
The Mk18 comes in both a 14.5 and a 10.3 inch variant, there is no designation in the title to differentiate between the lengths.
Peq-16 would not work for a mk18, Peq-16 is standard issue to the USMC, Peq-15 (and whatever the newer socom PEQ that is based off of the 15 is) are what you want for a mk18.