The magazine tubes for the three guns of this family are quite different.
First off, the SAS12 tube is much longer
than the LAW12 or SPAS12 magazine. No, the SAS12 doesn't hold more
rounds -- it's just without all the gas-operation junk, the length of the
tube built into the receiver is shorter on the SAS12. So it needs
a longer external magazine tube.
The ends are different.
As best I can deduce, towards the end Franchi started shipping SPAS12's with a plastic version of the magazine extension. This was an exact copy -- note the flatter end of the SPAS12 extension.
Then E-GunParts started selling an extension claimed to fit the SPAS12. It was from another Franchi gun all together. It was the length of the shorter SPAS12 and LAW12 extensions, but had a SAS12-style end on it. It can be used on the SPAS and LAW, but only with a spacer. They no longer sell it.