That is a nice adapter.
It has the large 4500 gasket area and the smaller 4150 footprint on the other side to clear linkages and even the 50cc accelerator pump housing.
And the tapered transition is a good feature.

It seems like the dual bolt pattern 4150 carbs have been gaining in popularity……so I’m surprised that Holley/QFT decided to discontinue this item.

I guess that’s why I’m not a marketing expert.

I've been making carb spacers for local engine builders for a number of years. I've been asked twice to make a 4150-4500 spacer. I haven't bothered making them a stock item for that reason.

Disclaimer: These are my measurements and are NOT intended as manufacturing specifications or any specific brand.

When putting a 4150 carb on a 4500 intake, there is only 1/4" gasket sealing area as shown below (5.25-4.75)/2 = 0.25
That might be enough, but is minimal. And if using a gasket with the 4150 center hole, the gasket will not be supported above most of the intake plenum.
An adapter plate improves both of these. In addition, a 50cc accelerator pump body extends below the carb's baseplate, and can contact the intake without a spacer.

Ours are open spacers with o rings...Not any issue sealing things. We make them oprn to be able to support oval blade carbs which work very well.