Remington 40X 22lr single shot/repeater?

I've just purchased a Remington 40X, 22 LR, heavy barrel, single shot, sporter stock with cheekpiece (no rollover), checkered bolt handle like a 700, Unertl BV-20 scope. There is not a mark on the rifle, the action is very tight, not broken in at all. I'd give it a 99%.

It has a hinged floorplate, the stock is inletted for a magazine, and the rifle appears to have a single-shot adapter bolted in place.

Serial is 05xxxxR.

What have I got?

Is a 22 LR magazine available for this rifle?

Tim