Hello again Xotl,
This is actually just a wording issue. The table's use of NA means "don't worry about the slots it takes up, because it's not a weapon, and DropShips don't use slots on non-weapon items)" rather than "you can't mount it; ignore the entry in Game Rules that says otherwise".
Thank you again for the reply, but I do not feel based on information I recently stumbled on that "NA means "don't worry about the slots" the weapon/item "takes up, because it's not a weapon.
The Inner Sphere Heavy Weapons and Equipment Table CGL TM Hardcover 2007WizKind Inc and PDF 2007-2018 the P(rotoMech) Space Column entries are annotated NA. On CGL TM p. 10 ProtoMechs are "a strictly Clan invention" and on p. 341 Heavy Weapons and Equipment Key Space Entry Code has "NA = Item may not be mounted on these units".
On the Clan Heavy Weapons and Equipment Table CGL TM p. 343 the P Space column has a number of NA entries. For most of weapons and equipment with the NA annotation the weapon and/or ammunition exceeds the tonnage for any of the ProtoMechs.
Based on the information provided the annotation of NA indicates that the weapon or item cannot be mounted on the unit. Predominately, units that cannot mount a weapon or item appear to have the annotation of NA in the Space Column. Weapons and items that cannot be mounted on Standard or Aerospace units also have NA annotated in those columns.
My apologies for still being unsure of what the table annotation of NA means even with your clear and concise explanation.
Tom Rux