T8 and T12 tubes have the same pins on the end, so in most fittings both T8 and T12 will fit.
8 ft tubes were all T12.
6ft and under were made in both T8 and T12.
T12 are rare now as very few if any places make them.
Quite what the extender product contains isn't clear, but presumably a small ballast in a T8 casing which operates in series with the ballast in the original fitting.
They only appear to have a single product, yet 8ft tubes and fittings were available in both 100W and 125W types.
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