Receiver-mount Pic rail for untapped 500A?

[Robo_Railer]

I've got an early-1991 Mossberg 500A that isn't drilled and tapped for a scope mount. I'd like to put a Picatinny rail on it for a reflex or holo sight.
Customer service at Mossberg sent me a reply Monday which said in part

The receiver can't be able to be drilled and tapped in order for us to do
that you would need to send us your complete firearm and we can change
it for you, We charge $75.00 , The thread size for 1997 is 6-48. Or they
sell saddle mounts so you can install it on the receiver so you can
install the scope, You can find it after market.

Um, okay.

Mesa Tactical makes a kinda cool rig that mounts on the sides of the receiver. One version of it also incorporates a SureShell 6-shell carrier. Unfortunately for my needs, they only make 'em for Remingtons.

Suggestions?

 

[Blubacon50]

Robo,

B-Square makes a receiver mount with a pick rail on top. It sits up high enough so you can still use the bead sights. Cabella's, among others sell it.

Blubacon50

 

[Robo_Railer]

Those seem to be what I'm looking for. (And they make them for an Ithaca 37, too!) The only bad part might be that "Picatinny won’t fit Weaver, but Weaver will fit Picatinny” thing.

Thanks, Blubacon!