In 1975, there was a severe civil war in Lebanon. Most parts of the country were ravaged. Thieves got into the ministry of PTT, and among what they stole are parts of 1974-75 issues of Lebanon. In 1977, after some peace agreement signed between the parties, the ministry decided to declare all previous invalid, and that what remain in the ministry stock to be overprinted by a grill. This grill is a proof that the stamp is not stolen. Lawful holders of the previous issues can not use them on post, they will be useful only for collections.
Note that not all stamp issues have been stolen, also there are stamps rare with the grill or without it.
Also there are differences between the grill on the same stamps (uncatalogued).
Here you have:
Three stamps remain in stock from this 1971 nice set, 45p, 70p, 85p. This is the 85p stamp. Look how the grill is very sharp.
The entire handicraft set minus one was overprinted (no 35p). The 70p without grill is rare.
Also the entire Lebanese costumes, 4 values was overprinted, here the grill is not very sharp. The 5p with grill is almost rare.