Those are the date an shift codes of the heads. Arrow pointing to D or N is day or night shift. I108 would be Sept 10th 1968, I128 (9?) Sept 12 1968, or 69 I can't quite make out the number. 69 350 should have #47 or #48 cylinder head.numbers are cast into the center exhaust port.
Maybe a few pics of the block numbers will help you find them. One pic shows my block on an engine stand, you can see the 428 cast into the block between the center and the rear core plugs on driver's side, it is cast into the passenger side between the center and front plug just behind the dip stick tube. Yours should have 350 cast into the same locations if they used it in 69. hard to see when in car but a flashlight and a mirror will help.
In pics re assembly 10 and new build 105 you can sort of make out the position of the numbers near the distributor hole. Pics top end 25 and 26 are close ups of those numbers. The number 9792968 with the sideways P is the number just below and behind the passenger side cylinder head. The P is Pontiac division and the 97... is the block identification number. The number behind the oil passage plug shows the date of manufacture, November 30th 1968, the arrow pointing to N just behind the distributor hole shows it was made on the night shift. the 15 indicates it was shift 15, foreman's name was John Peterson, lead hand Bob Logan and they just finished lunch. Well, maybe not. I forget what the 15 is.
Pics re assembly 1 and Mad Mans 33 show the location of the engine number stamped into the block just below the passenger side cylinder head. These can be hard to see if the stamping was light and there are a few layers of paint and or crud.
