Just to pick nits, The horizontal lines were generated when a broach was run through the frame window. The uneven nature of it could be due to chip build up between the cutter teeth (the chips work harden so are able to scratch the parent metal), a worn broach, or grit in the cutting fluid. Or any combo thereof.

You'd be hard pressed to generate those marks in that direction with a rotating cutter!

Sorry, machinist nerdery.