Charter Arms used to advertise the Undercover - way back in the day - as "the lightest steel-frame 5-shot .38". The "frame" has always been a 2-piece arrangement, consisting of the cylinder frame and the trigger guard/grip frame. This was part of the design to make manufacture less expensive.
The Undercover - the original Charter Arms design - used an aluminium trigger guard/grip frame and a steel cylinder frame. The latter contains obviously the cylinder and barrel, and also the lockwork (trigger, hammer).
The subsequent Undercoverette (6-shot .32 SWL) used the same frame arrangement. The later Bulldog used a larger frame, but of the same 2-piece design, construction and materials.
The material used for the grip frame has varied over the years. When Charter introduced a polished stainless 3" Bulldog many years ago, it used a steel grip frame. The steel grip frame is also used on their current "high polish" models, including a .38 Undercover and the .357 Professional IV. These are amongst my favorite models. Thee additional weight of the steel grip frame makes handling and shooting very nice.
The cylinder frames are usually all steel, except for the "Lite" models which are aluminium.
Charter began using a polymer mould for the grip frame on some models about 10 years ago. See the quote above concerning which models have polymer or aluminium grip frames. I guess cost was the reason. Also, as originally designed, the guns had exposed ejector rods, like the older Colts. When Charter began enclosing the ejector rod with a shrouded barrel, the overall weight of the gun went up, and polymer being lighter they could still claim the "lightest steel frame 5-shot .38". I have a DAO 38 Undercover with an early polymer grip frame and it makes for a lighter gun and it shoots right to the sights. Aesthetically, the earlier polymer grip frames left much to be desired.
I also have the Professional 7-shot .32 H&R is on the "Large" (Bulldog) frame and has the aluminium grip frame
I just picked up the new Undercoverette in .32 H&R Mag which is now a 6-shot built on the same small frame. It has the polymer grip frame. Out of the box the finish of the steel and the trigger pull are excellent. The color of the grip frame is matte silver and a slightly contrasting but good match to the steel parts. Notably, the fit of the grip frame to the cylinder frame at the top of the tang is vastly improved, with no sharp edges of the steel cylinder frame protruding into the shooter's hand.
However, you can see the seam of the mould line on the grip frame tang, and with the grips removed there also is a slight moulding flaw visible at the front base of the frame. Given the modest price of these American-made guns, this is not much cause for complaint, and doesn't interfere with function.
I replaced the rubber-like full size grips, which feel great, with a set of the smaller Charter wood grips. I hope to shoot it this week.