From the email "There are a few years that the Holley 4150 came with 2 extra adjustment screws on the GM vehicles. These screws directly impact the amount of air allowed into the idle circuit and you will notice there is no idle air restriction bleeds pressed into the main body." Those deal with idle fuel I believe and....
Your thinking of air if I'm not mistaken.
Seen other methods of air bypass to help idle, most just drill the throttle blades if needed.
I have seen in a lot of Holley boos and researching and stuff and if you look at the choke tower and the way the numbers are put on them it has to be something from the 60's like an oem carb and if you notice there are staked screws holding the choke flap in place and Holley has not used that design since the 70's and did away with that style and went with the modern style.
I think you locked in on time period, as i recall a three digit date code is the 60’s, But the photo from Allstate has a 4 digit code indicating after 1969. Could be an over the counter replacement In Allstate photo.