It has the later style distributor cap which I guess is the B cap.
Copy that.
FWIW, I would not do a Compression test as a first step. (More on this point a bit later)
Backfiring has a schedule of probable causes. Despite what you may have been told, first among these is over-advanced Timing. Next is Mixture, and later followed by possible Valve problems. Last on the list is inadequate cooling.
The fact that the car's behavior changed under operation is making my Spidey-Sense tingle toward it being operator error. One has to realize that these motors are 'tuned' in real-time by the driver, which is totally unlike a post '96 or even a post '80 engine. Many of the old cars that come my way get fixed by putting the base adjustments back where they are supposed to be and then training the owner on how to drive it correctly.
It is unfortunate that basic IC Gas Engine tech is no longer taught. These days it's all 'Read the Code; point at the Part and blame it'. (EG: Code P1110 probably means the vacuum hose is bad, NOT the Turbo Bypass Valve).
Sorry if this seems like a rant but it is just far too often that I see this approach taken by a person who doesn't know why the thing worked in the first place, did something, had a buddy do something else and finally brings it to me to figure out what's REALLY going on.
From where I sit, the only way Compression can cause a backfire is because someone altered one of the above as a method to compensate for poor compression, whether real or perceived.
As far as checking compression goes, there is a relatively new device called a 'Leak-Down Tester', which I always use BEFORE checking compression. The leak-down test often makes the compression test unnecessary except as means of verification.
