Bach 1-1/2c threaded with Standard Bach threads. Stock 27 throat and back bore.
Modification done by Karl Hammond of Hammond Designs
http://www.karlhammonddesign.com/products-karl-hammond-design-custom-instruments-mouthpieces.php#
For best fit, find a mouthpiece with a rim you like and send it to Karl Hammond along with this bottom half so that he can properly align everything.
https://alphavariable.com/collections/trumpets-and-mouthpieces