It's surely a custom part. I've only seen JBL, Altec Lansing use interstage transformers in power amplifiers.
Today it's all ferrite materials tailored for SMPS applications. Those cores give high distortion in audio use.
The hard part is finding the pot cores.
Many years ago I searched for "telecom grade" ferrites for a Pultec Eq clone project.
My junk box shows them to be MMG-Neosid P11 material and I thought the B-H curve is fairly linear for low distortion.
Perhaps ask
Sowter Transformers as they reverse-engineer and manufacture vintage audio transformers.