I am pulling a lot of information together here, making a few assumptions, a few guesses. So, one, don't make any bets with anyone about the information I have here and, two, let me know if you have any facts yourself along these lines.
When Franchi first conceived the SPAS it likely looked like this:
Note the very short barrel with no more room than a normal magazine cap under it. This was their design. Unfortunately it had a barrel length of less than 18", so it could not be sold (easily) in the USA. It also was kind of short in magazine capacity for the type of gun it was.
So the first gun they offered in the USA (and the one they still show in their brochures) is this one:
Note the barrel extension. This looks a heck of a lot like the Barrel Extension Accessory they still sell to extend your barrel even further. Note the extension has the same thread protector cap on its ends as a barrel does. Note the entire extension is this large diameter so that it can have threads in its tail end to screw on the end of the barrel. To be legal, this would have had to have been permanently affixed somehow. I don't know what they did and may never know because, outside of these sales brochure photos, I've never seen a gun like this one.
The first SPAS's to hit the USA looked
Note there is still some sort of barrel extension, but it necks down right after the thread protector cap. It is permanently affixed to the barrel end by welding or soldering. Note it is even longer than the extension in the previous photo. Also note the front sight location, still at the end of the original barrel. Also note the new space for the much longer magazine extension.
The last version to be shipped into the
USA was this one:
The barrel is just a regular barrel made longer. Note the sight all the way at the end of the barrel.
In the UK, where you can still import and buy these guns, they are required to have 24" barrels (and 40" overall length). Click here to see a photo of this 24" style.