"reptiles usually also have a muscle ‘supinator longus’/’tractor radii’ sensuHolmes (1977) (see also Russell & Bauer, 2008), which is actually the probable homologue of the mammalian brachioradialis"

Also, I'd guess it is from pterygialis cranialis pelvicus, it is innervated by preaxial nerve roots (C5 and C6, same as musculocutaneous nerve that innervates pterygialis cranialis derivatives) and it is positioned preaxially, the tendon attaches at the preaxial side of the radius, the side that lost most appendages (whereas postaxial side kept them, the fingers. )

Synapses

Abdala, V., & Diogo, R. (2010). Comparative anatomy, homologies and evolution of the pectoral and forelimb musculature of tetrapods with special attention to extant limbed amphibians and reptiles. Journal of anatomy, 217(5), 536–573. https://doi.org/10.1111/j.1469-7580.2010.01278.x

Holmes R. (1977). The osteology and musculature of the pectoral limb of small captorhinids. Journal of morphology, 152(1), 101–140. https://doi.org/10.1002/jmor.1051520107