"TCOT Silent Partner" plays very much like a B-movie film noir, with gangsters, gun molls, drunks and dames. And it's an unusual Perry Mason episode since it features no courtroom scenes. Perry even sidesteps Paul Drake and does a lot of his own sleuthing in this show, even discovering the corpse before the police.
It throws Perry Mason into a kind of cop/PI role rather than as a lawyer (Hamilton Burger gets only one scene this week, but it's a nice face-off with Perry), but the mystery hums along at a good pace. And it offers more examples that contradict the idea of Mason as a "straight arrow" - far from it, he'll do whatever he can to protect his client, even coercing her into firing a shot from the murder weapon to discredit any tests the police could run.