That 'crack' in the Star PD frame and a lot of other Star frames is a relief cut the factory made in the frame thru the thin portion of the slide rail into the slide stop cut-out.
The frames did crack right there on their own before the factory started doing the relief cut.
Then some of them continued to crack thru the lower edge of the slide stop cut-out.
Flexing of the frame was occuring I guess. Especially on the Aluminum framed pistols.
The BK 9mm was prone to this.
Some of them even cracked on the other side (RH side) where the disconnector bar cut is milled into the frame farther back by the hammer.
I don't know of any thing that was done to prevent that issue.
The slide stop 'cut' is often seen as a frame crack when first observed by the pistols owner.
A clean straight cut with ample clearance is a factory done relief cut.
Earlier guns made before the factory placed the relief cut into standard production guns may show an actual crack.
I've seen some of these early guns with an extra aftermarket very small hole drilled through the frame just below and intersecting the bottom of the slide stop cut-out.
This attempting to halt any further cracking downward on the frame by giving a crack a flexing end point.
It's a common 'fix' to prevent cracks in things like semiauto shotguns recvr's from traveling any further than they already have.
The Star PD was the must-have CCW pistol back when.
Lots of L/E purchases. Most liked them
The last one I bought was a 'parts only' pistol that somene of questionable thinking and hands on ability had decided to make the PD even smaller than it was.
They had cut the frame grip length shorter, even too short for the main spring to work. Being alloy there was limited ways to make things whole again. Grips cut as well.
Of course the orig pricey magazine had already been shortened..
The top 1/2 was in tact so that was the valuable part.,,and I resold that easily.
The nicest Star in 45acp I've had was a Model PS. The 1911 lookalike.
Model P,,The 'S' signifys a magazine saftey on Star pistols.
An x- Guatemala Fed Police pistol in excl contition. A small lot was imported in the late 80's early 90's.
I was in my Star pistol accumulation phase back then.