Nice write up on a great pistol.

There was one design change on the Model BM towards the end of of production. That was to change the 'positive ignition' firing pin to the 'inertia firing pin' type.

Most all the STAR semiauto pistols had the positive ignition style up to that point where with the hammer down, the firing pin tip protrudes from the breech face. That made it unsafe to carry w/a loaded round in the chamber and the hammer down. A dropped pistol or a hit to the hammer would/could fire the gun.
The inertia style firing pin is simply a 1911 style where the pin is shorter than the breech and rebounds under spring tension after hitting the primer. A hammer down on the inertia style will not protrude from the breech face making it safe to carry as w/a round in the chamber.
Some still dispute it is safe to do so,,the reason Colt installed the Series 80 firing pin block in their 1911A1's.
But that's the story on that anyway.
Easy to check for. Pull the slide back and lock it. Push the back end tip of the firing pin iin till it's flush with the slide abutment and hold it there. Then see it the tip is poking out from the breech face.
If it is,,it's the Positive Ignition Style
If not,,it's the Inertia Style firing pin.

Th captured recoil spring and guide was a new feature from STAR and introduced on the Model BM.

That little slot in the frame that slices downward from the rail and into the slidestop cutout is specificly there to avoid a stress point.

The extra CAI Ser# on the gun is part of the Import Marking regs.
The Fed regs read that the Imported cannot use the manufacturers applied ser# if that number is duplicated on ANY other firearm that that importer brings in/sells.
With the thousands of firearms they import, and possiblilty of one of the Ser#'s being duplicated somewhere, sometime on something,,CAI and some others have gone right to slapping an Importers Applied Ser# on the pieces.
They've showed up on several different CAI imports much to the disgust of customers as they look at a perfectly good mfg'rs stamped number on the same gun.

It's a CYA by the importers I suspect,not knowing if the Murphys LAw of chance will come back to bite them or not.
The Importers applied ser# becomes the legal ser# of the firearm as far as transactions in this country are concerned.
The Fed reg about altering, removing, obliterating a manufacturers applied ser#,,blah, blah,,,now includes '..or Importers applied serial number..' to the language.

It's most likely easier for places like CAI to inventory as well, They run the items through Dottie the dot matrix marking monster machine and they all get the import markings AND a new ser#. Very convenient for a computerized program to inventory, sort, ship and keep records of.

Those Spanish Police and Armed Forces inventory numbers have always caused problems in the past.
It was CAI that logged a large number of Model B and Model B Super pistols imported in the 90's and used the Spainish Armed Forces Inventory # as the ser# instead of the real ser# on the pistols.
Maybe that was an early lesson in all this.

Those are good prices on the spare magazines, Anything in the $25 range is good.
The last time these Model BM pistols came in, the extra magazines quickly dried up and prices went to $50 and above,,and that was 15yrs ago or so.

Proof date code (triad of 3 proofs) is probably under the left grip on these later made STARS.
One of the proof mark is the date code and will tell you when it was proofed.
Star Firearms: Dating your Star Pistol with Proofmarks
Ser# charts are avail for production as well as you have found.

If you want to remove the magazine safety feature,
Remove the grips,,** make sure each grip screw is accounted for as to which placement and grip hole position it was removed from. They go back in the same positions as they came out**.
The mag safety is implanted on the right side of the frame along the backstrap w/an intrgrel pin that goes thru to the left side of the frame.
Simply push the pin out from the left side. The mag safey spring will come out with it (all one piece).
They usually come out easily as the grips are the only thing that captures it in place.

Once removed,,the magazine safety feature is removed. You are removing a factory designed and installed safety feature,,be aware and take note of that. Make sure that is what you want to do.

The magazine will be a drop free feature. If the mags still do not drop freely, check the grip screws once installed and make sure they are not too long and extending inside the mag well riding on the mag body.

Can't hardly buy an Aimpoint for the price.