Here's a thought...
It is possible that the Hawken shop replaced or rebored/rifled the barrel of a customers existing rifle, there were many plains rifles similar in style to the Hawken being produced both in St. Louis and back east in the mid to late 1800s. They, the Hawkens, were doing quite a bit of that sort of work during the gold rush as stated by Neb Roberts and as quoted in "The Plains Rifle" by Charles E. Hanson Jr. (page 43):
"I have been told that back in 1849 when the first rush of gold seekers started to California, the Hawken shop recut many of the small bore rifles these pioneers had and made them from 42 to 50 calibers, depending on the size of the barrels.
It is my opinion that the Hawken you mention having locks like the Kentuck rifles. were some of those rifles that Hawken converted to his plains rifle type, instead of being rifles that were originally made by him. Of course when the Hawken brothers were were 'rushed' with work for pioneers who stopped over in St. Louis to get their rifles, had many different types of locks, because Hawken just recut the barrel to larger size and used the rest of the rifle in its original condition, including the lock, nipple, etc."
If just the barrel was replaced or rebored/rifled in the Hawken shop would it then have the Hawken stamp? I don't know. Also, the rifle in the current auction is 54 caliber, a bit up from the 50 cal Ned Roberts mentions.
dave