Understock for "tree rose" version of Petite Knock Out

Does anyone know what understock would have been used on a “tree rose” version of ‘Petite Knock Out’?

This photo is from a local Lowes hardware store. Note that the understock has sent out long canes, overtaking the actual budded rose. I don’t have an interest in ‘Petite Knock Out.’ On the other hand, I have quite a bit of interest in understocks for budding/grafting. Any idea what this understock is?

Please note that it does NOT appear to be R. multiflora, which I initially thought it might be.

Welcome back, Matt! I’d imagine that’s likely Dr. Huey as it’s the most common stock used for commercial production in the US, that would be my guess. They use it for all the other standards, why not this one?

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Gotta be Dr. Phooey.

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