I used to have something exactly like this 40 years ago.
Spiratone I think the brand was ... and a whole $40 plus shipping.
But T-mount? who uses that anymore?
FlyingDiscT-Mount is still commonly used to connect DSLRs to telescopes. You need a camera-specific T-ring to attach it. Makes sense this type of lens would use it. Making multiple versions for each brand would raise the cost significantly.