The brace should be on any 69 dash with a center gauge (clock or fuel). It's needed for stiffness.
This brace thing is a puzzler ,at least to me. I found it in the AIM pg. M35 A10. Yea, that is the first place I look for instrument cluster parts is in auto trans section (sarcasm)!?
My BB car has a clock, no tach. It did not have a brace, at least when I took the car apart. I bought a cluster eons ago because the radio hole was uncut to replace the butched OE one in the car. It was a non-clock, non-gauge, AC piece with a brace. Recent parts car has a decent cluster, again, non-clock-gauge car. No brace there either which aligns with Kurt's assessment.
To summarize: 1)OE-w/clock-no brace,2)no-clock, w/AC-had brace,3) no clock-no brace. If I get a chance I will check out the other 2 cars...it may be awhile before I check, this old man does not stand on his head looking up at dashes too well anymore.
On a side, I was sweating bullets cutting out the clock hole and rear defrost hole on replacement dash I am using. It turned out real well. I even transferred the old dash sticker onto the new unit and replicated the insp. crayon mark.