Yep. Most of your private label parts are built by an existing company and rebranded; I doubt Summit manufactures any of their house brand parts in-house. Some other examples of Summit parts and who makes them:
1. Proportioning valves - Willwood
2. Wideband systems - Innovate, though the gauge is probably sourced elsewhere.
3. Intake manifolds - A variety of sources, including Weiand and Professional Products.
It does seem weird that these carbs don't have a counterpart elsewhere; perhaps they bought a design Holley didn't want to sell under their own name.