I believe stew is correct! I have never seen a BBC TB in person. I find from a little searching that a BBC TB should be 645 CFM and the Holley one here is rated at 670 CFM, so not much difference.

The 4.3, 5.0 and 5.7 TB measures 1.680" on the bores.

This Holley unit measures 2.042" on the bores.