Here is a photo of the 109 block. The SG1 cast on the right side of the block indicates that this block is a 109 block that was a Buick factory replacement 109 block or Turbo Trans Am block.
There are 2 obvious visual items that will immediately tell you if it is a 109 or SG1 block.
One is the turbo drain back hole, or boss for a hole, near the top front pass side.
The other are 2 block drain plugs on each side of the block just below the deck at nos. 3 & 4 cylinders.
We have used many early blocks and have experienced no difference in strength and reliability between those and 109 blocks.
I installed an early block in my race car while my alum block is being freshened to determine how many 9 sec. passes it will take before it splits in 2, or the decks crack, like so many people have predicted?
After over a year of dyno pulls and many passes it is still doing its job!