Many T2 mounts and Vivitar lens converters have a numbering system to identify the camera lens mount fitting. This originated from Taisei Kōgaku the originator of the Tamron lens system. Vivitar also used the numbering system for their converters, so if you see one with 2x-4 you know it’s a 2x converter with a Canon FD lens mount.
Below is a comprehensive list of the numbering system for the T2 mount and the 2x and 3x converters of Vivitar. Other lens manufacturers may have adopted the same system. The ones in bold are confirmed as existing, others are discovered from researching various web sites, but I’ve never seen an actual example for sale.
TA-1 / 2x-1 / 3x-1: For M42 screw mount camera bodies such as Chinon, Cosina, Praktica, Zenit, early Asahi Pentax, Yashica and Fujica
TA-2 / 2x-2 / 3x-2: For Exakta camera bodies, and some Topcon and Praktica models with same mount.
TA-3 / 2x-3 / 3x-3: For Nikon F/AI cameras bodies.
TA-4 / 2x-4 / 3x-4: For Canon FL/FD camera bodies.
TA-5 / 2x-5 / 3x-5: For Minolta SR /MC/MD camera bodies.
TA-6 / 2x-6 / 3x-6: For Miranda (M42 screw-mount) camera bodies.
TA-7 / 2x-7 / 3x-7: For Konica AR camera bodies.
TA-8 / 2x-8 / 3x-8: For Konica F camera bodies.
TA-9 / 2x-9 / 3x-9: For Miranda Sensorex Series cameras
TA-10 / 2x-10 / 3x-10: For Leicaflex
TA-11 / 2x-11 / 3x-11: For M39 (LTM/Leica) screwmount camera bodies such as Leica, FED, Zenit, etc
TA-12 / 2x-12 / 3x-12: For Olympus Pen F / FT
TA-13 / 2x-13 / 3x-13: For Alpa Reflex camera bodies.
TA-14 / 2x-14 / 3x-14: For 16mm C mount
TA-15 / 2x-15 / 3x-15: For Topcon / Exacta camera bodies
TA-16 / 2x-16 / 3x-16: For Zeiss Contarex camera bodies.
TA-17 / 2x-17 / 3x-17: For Topcon Auto 100 camera bodies.
TA-18 / 2x-18 / 3x-18: For Zeiss Icarex camera bodies.
TA-19 / 2x-19 / 3x-19: For Kodak Retina Reflex
TA-20 / 2x-20 / 3x-20: For Olympus FTL
TA-21 / 2x-21 / 3x-21: For Olympus OM camera bodies.
TA-22 / 2x-22 / 3x-22: For Pentax K bodies and Chinon, Ricoh etc with same mount etc
TA-23 / 2x-23 / 3x-23: For Fujica AX camera bodies.
TA-24 / 2x-24 / 3x-24: For Contax and Yashica camera bodies with C/Y bayonet mount.