906 or 915 ?

Here’s one from about 25 years ago:

446
Trw L2266’s
Bowl blended 906’s, 2.14/1.81
Torker with Demon 650
General Kinetics version of a Comp 280H
1.6/1.5 rockers
1-7/8 x 3 headers

View attachment 1716016184
Impressive stats,,, stupid question here…. Sorry… what exactly is bowl blended???