What is this strange old tool?

My dad found this thing where he works and he brought it home to see if I knew what it was. The part that looks like a flat head screwdriver with a t handle can rotate and slide back and forth. The writing on the handle says "monarch - - - - - - phila 34 pa - - - - - - 203

Thanks in advance.

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It is a valve lash wrench.

Here is a newer one.
The difference between the old and the new is serious cool beans.
And not just one or two.
In England a Valve Tappet adjuster. :-)
Is tappet what they call rocker arms there?
Technically the tappet the screwed bit on the end of the rocker arm resting on the top of the valve stem.
monty324 (author)  rickharris1 year ago
Thank you very much.
monty324 (author)  Josehf Murchison1 year ago
Thanks for the help.
You are welcome