Cemetery tombstones

Does anyone know the significance of these tombstones?

"R.I.P. / 7/20/49 / Quiet Isn't it?"

"En Memoria / The Recordamos / In loving memory / 12/16/86"

"10-67"

The last one is very odd, just a tombstone that simply reads "10-67"

Any thoughts or ideas?

I was going to say cole phelps but he died on sep 30 1947.

Oh... I don't know why, but I mixed and mashed up the cemetery section in GTA 5 with Max Payne 3's cemetery section, so I was going to make a comment on Max Payne 3's tombstones. Anyways, I don't know.

Half Life 3 confirmed

10-67:

10 from 67 = 57

57, 5 + 7 = 12

12, 1 + 2 = 3

Verified, Half Life 3 confirmed.

Probably some call outs to people the developers knew. Paying their respects, so to speak.