I am not familiar with Mowrey so I spent a little educational time Googling it for personal interest and there isn't much is there? It is bicentennial so of course that would suggest it was manufactured in 1976, but the only thing I found on Google was a past sale on Gunbroker where an auction house listed and sold one in 85% condition for $350. There was only one bidder.
There is a reference to Mowrey Gun Works Inc in the Blue Book of Modern Gun Values Third Edition (I don't have a newer edition). I do not find the 1776 Bicentennial 690 you describe, however. Of the few models they manufactured, most had an MSRP of $345 and available only as a kit. The exception was their 1:30 Twist Conical Rifle that was not specified as a kit so I assume it was available already finished. The following is a narrative of Mowrey Gun Works, Inc. (keep in mind, this may or may not be the same Mowrey manufacturer as your gun but the way it is spelled is somewhat unusual, so maybe this is it):
Current manufacturer and distributor located in Waldren, IN. Previously manufactured in Saginaw, TX. Previously distributed exclusively by Mountain State Muzzle Loaders Supplies, Inc. in Williamstown, WV. Dealer and consumer direct sales. Mowrey Gun Works has recreated the guns designed by Ethan Allen and marketed under the name Allen and Thurber in the early and mid 1800s. The models themselves are beautifully hand crafted with "cut rifled" brown barrels (each groove cut individually using as many as 20 passes) and actions using only five moving parts creating exceptional accuracy and reliability. The 1:30 in. twist rifling was designed specifically to stabilize conical bullets. Each specimen is available with a number of features and options (listed below). Mowrey Gun Works standard rifle features include: curly maple stocks and forearms, front blade-buckhorn rear sights, hand rubbed finish, and brass or brown steel receivers. Options include: premium curly maple, cherry or walnut stock and forearm, custom barrel lengths from 22-40 in., primitive fixed sight, target sights, and Scheutzen style buttplate. Beginning 1994, Mowrey Gun Works also began making reproductions of Hopkins & Allen underhammer rifles.