Rich,
Interesting thread. Thanks for posting the photos. In my humble opinion, the "S. Gross" rifle is a re-barreled rifle made by Samuel Loudenslager from Center, Juniata Co., PA. Here are my reasons for coming to this conclusion (or hunch)...
1) I have seen several other rifles signed "S*L" with nearly identical patchbox, engraving, triggerguard, buttplate, sideplate, upper and lower ramrod pipes, inlays, incised mouldings, cheek rest, buttstock profile, and lock panels. All attributed to Samuel Loudenslager.
2) I have never seen another Samuel Gross attributed rifle with any of these same features.
3) The fathers of both S. Gross and S. Loudenslager were from the same geographic area (Union/Snyder Co.,PA). It's quite likely that the families were even acquainted with each other. However, I'm not aware of any records which indicate that S. Gross and S. Loudenslager ever worked together or lived near each other.
4) Samuel Gross moved from Juniata Co., PA to Seneca Co., OH in 1831 when he was about 21 years old with his gunsmith father and gunsmith brothers. Census records indicate that Sam Gross lived continuously in the Bloom Twp., Seneca OH until his death in 1894. Sam Loudenslager was born in 1812 near Selinsgrove in Snyder Co., PA. He bought a farm and set-up his gunsmith shop in Juniata Co. near Mifflintown sometime during the period 1840-1850, at least ten years after Sam Gross left for Ohio.
5) The "S. Gross" rifle is equipped with a patent breech. This is indication that the barrel was probably fitted to the stock sometime during the late percussion period >1850.
6) During the mid 1850's many Upper Susquehanna families were moving West and settling on inexpensive land in Northwestern Ohio in and around Seneca County were Sam Gross worked. It's quite possible that someone carried a Sam Loudenslager made rifle out to Ohio and for one of many plausible reasons went to gunsmith Sam Gross to have the rifle fitted with a new barrel.
I'm hoping others will jump in here with their opinions.
FYI... there are a couple nice Henry Groce (Gross) Sr. longrifles featured in an article by Dr. Whisker in an AOLRC newsletter. Here's a link...
http://www.aolrc.org/wordpress/wp-content/uploads/vol-xxiv-no-1-aug-2001.pdf Henry was the father of gunsmith Sam Gross. These rifles possess many of what I consider to be classic Union/Snyder County features. Both guns were likely made before Henry and his family moved to Ohio.
Thanks again for starting this interesting discussion.
Mark Loudenslager