Jeff I agree, I have only seen 1 4BC Damper used for '68-'69 4BC.
Here's a compiled list of Pt #'s that I believe to be most if not all factory HB's
The first image is the 1BC with the last 4 showing a VHTF 4BC.
At this point, any/all dampers are hard to come by, but IMO the '68-'69 4BC has to be the hardest.
Crankshaft Damper;
9783090 GM ’66 1BC
9782289 GM ‘66 4BC
9783089 GM ‘66-67’ 1BC
9778553 GM ‘67 1BC
9778557XR – 9783100XR GM 66-’67 1BC HB/DAMPER PULLEY
9778551 GM ‘67 4BC
9792484/9792478 GM ‘68-‘69 4BC
9792538 (250) 4BC
9791717 GM ‘68-‘69 1BC
9791723 GM ‘69 1BC
9792984 GM 68-69’ 4BC
Cheers.
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When I die, I want to go peacefully like my grandfather did, in his sleep.
Not screaming like the passengers in his car. 
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