From Mossberg Website
#50668 590® 12 gauge Pump Action w/ Speed-Feed® synthetic stock, Ghost Ring Sights, 9-shot capacity, 20" cylinder bore barrel, parkerized finish.
#51663 590 A1™ same as #50668 pictured above, except features heavy-wall barrel, metal trigger guard and safety button and field synthetic stock.
From Wikipedia
Model 500 vs. Model 590
The variants with the extended magazine tubes use different barrels, as the barrel is held in place by attaching to the end of the magazine tube. These eight shot models are referred to as Model 590. The Model 590A1 is a heavy-duty 500 with a steel receiver, rather than the aluminum alloy receiver found on the other 500 models, plus a steel trigger group (vs. plastic for the standard models) and a thicker walled barrel. The 590A1 is available in six and eight shot models, with standard and Bantam length stocks.
The shortest, and in fact only barrel length available for the eight shot model is 20 inches, which fits flush with the long magazine tube. The shortest barrel for civilian models of the six shot model is 18.5 inches, while military and police users can also get a 14 inch barrel (the 590 Compact), which is flush with the six shot model's magazine.
Hope this helps
Ochiix