There have been several odd runs of two toned guns that showed up years back.. No way of telling for sure what you have.
Here's a suggestion, call S&W (1-800-331-0852) hit the prompt for Customer Service (I think it's 2 these days)
When you get a customer service rep, ask them to provide you a SKU/Product Code for your gun, you will need to give them your serial number.
Once you have that, it may be something in the Standard Catalog, or if it matches a known model that might be a clue.
Not a 100% way to do it, but should get you closer. Ultimately, if it's an unknown sku/product code, you might want to get a factory letter to confirm how it was shipped.
Worth a little more digging, I have found the CS reps to be very helpful if you chat them up some... Normally they will provide the sku/product code, sometimes they will even tell you which distributor the gun went to, and the month/year it shipped.
Anything is more than you have at this time.
In any event, you have a very desireable auto, it looks good, 40 S&W is a great caliber, the DAO action and alloy frame make that a great shooting/carry gun.
FYI