Here's a picture of the forearm on my Kuntz rifle. Starting at the forward end of the lock molding, the forearm is an oval, and as you arrive at the entry pipe, it is flatter and flatter, but still rounded along the bottom. The upper forearm is virtually flat from the thickest section, which is about 1/8" thick about 1/4" down from the barrel channel, to the rod channel and pipes. Many of the Lehigh rifles I've had the pleasure to study are like this, especially the upper forearm, but some have a very gentle roundness to them rather than flat like the Kuntz. At the nose piece, there is a transition to a more rounded section, and the nose piece has a groove for the rod. This is not my idea...it's how Jacob made his rifles.
The upper forearm on Lehigh rifles is often much broader than rifles from Lancaster, Berks, etc. For example, there is almost no wood along the barrel of a JP Beck rifle by comparison. Even so, Lehigh rifles are not clumsy or chunky here.