To answer your question...
K & Co. not only held military contracts but were one of the largest musket contractors during the Napoleonic Wars. As far as I know, these arms were not marked with their name. Their Volunteer muskets...made for what was effectively private sale to volunteer units usually were marked. But...the crown and GR does not mean government purchase. It was commonly used on a variety of arms, including export guns and has no real significance regarding usage.
As to your lockplate...if I had to guess I'd say it was from the volunteer period, roughly 1795-1804. Depending on the size it could be for a musket or a rifle or even a pistol. Huge numbers of arms were produced for the Volunteer forces and they did not have to follow the Ordnance patterns precisely. Thus we see muskets with flat locks etc... Volunteer arms are nearly always of relatively high quality, often better made than their Ordnance cousins. They were subject to Ordnance Office inspection so they are never below the Ordnance standard. It was conventional for these arms to bear the Royal Cypher but they would not have had Ordnance proof marks or Ordnance Storekeeper's marks.
