Need a Remington expert

[Whit1072025]

I picked up an 870 youth magnum model at a gun show to build for next spring.  The gun works fine and has shot well thus far, but I looked up the serial number and it has a  W suffix  which indicates a 16 ga small frame 2 3/4 according to what I read.  The barrel is clearly a 20 ga 3 inch.  I was unaware they even made a 16 ga Express

Is this safe to shoot?  Does this really mean the receiver is different from the barrel?

Any help would be appreciated

Tried to post a pic but file too large