There are technically 4 Beretta 390 series shotguns that I am aware of.
The two main differences are "field" 390s and "sporting" 390s. The sole difference is the shape of the receiver, the field gun (plus Skeet and Super Skeet) having a slightly squared off rear receiver, somewhat reminiscent of a Browning A5 in appearance. The sporting receiver is rounded, similar in appearance to a Remington 1100 or a Browning Gold.
The A390ST and AL390 sport differing amounts of alloy in the components of the gun itself, with the AL390 being slightly lighter (I'm talking scant ounces here). From outward appearance, you cannot tell a A390ST from an AL390 without reading the designation on the receiver. Both of these models came with either the squared off or rounded receivers.
That being said, WallyWorld carries only 390 series guns, as far as I have seen. Theirs is a discounted version of the 390 field gun with a composit stock and modified Mobilchoke.
Bud