Sorry, can't help you on the drawings...
Looked everywhere, and like you, only found drawings of j's and k's
The only explanation I come up with for the lack of a primer button is that it's cheaper to make.
Cobras are available with primers as well.
Impco classified the primer as a diagnostic tool, and thinks that it's no longer required...
As for the schreider valves: I've never seen that, so i doubt that it's an Impco item. We just plumb guages into the ports for testing.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.