Hello migrantriver and welcome to the world of Pentax DSLR photography!
What you got is in all likeliood an Adaptall-2 lens with a custom mount for Pentax K-mount lenses (P/K). There is also the chance that you have got a lens for the Pentax KA-mount lenses (P/KA). Other than the engaving on the mount, you can tell the difference from the very look: P/KA mounts have lectrical contacts while P/K mounts have none. Take a closer look here:
Tamron Adaptall and Adaptall-2 Custom Mounts
Assuming that it is indeed a P/K mount, your lens will effectively be a fully manual Pentax K-mount lens. How to operate that, you can see here:
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html https://www.pentaxforums.com/forums/pentax-beginners-corner-q/60081-usage-green-button.html
Do note, that you will have to operate your K100D in manual (M) mode and that you will have to set "Aperture Ring Permitted" in the Camera Customs Menu.
It can be very rewarding to use these vintage Tammron Adaptall lenses on a modern DSLR. May I suggest that you browse the Adaptall Club pages in this forum for inspiration:
https://www.pentaxforums.com/forums/lens-clubs/59769-adaptall-mount-club-tamron.html
Congratulations with your purchases - and enjoy!