Occasionally, you'll run across a piece of cast iron that appears to have markings that have all but worn off, even when the rest of the pan seems to be in pretty good shape. Other than by erosion from pitting, the markings cast into iron ware, usually incised, don't wear down appreciably from use. What you're seeing may be "ghost marks".
Ghost marks are not all that uncommon in older cast iron ware. It seems most collectors have a story to tell about at least one crusty pan that emerged from the lye bath or electrolysis tank only to reveal a "ghost".
Ghost marks are what can happen when an existing pattern used to make the molds to create cast iron pieces is modified. If a foundry decided to change its markings, or remove markings altogether in the case of unmarked goods, they often did not go to the trouble and expense of creating new patterns. Instead, the old patterns had the obsolete markings filled in, and new markings added.
Over time and the repeated making of molds, the filler material would erode away somewhat. The result was that faint impressions of the old markings would be cast in pieces subsequently produced. If quality control didn't catch them, some pieces would end up being shipped out to market. Consumers at the time must not have been all that concerned, especially if they purchased an unmarked skillet only to find a name brand ghost mark on it.
Ghosts may appear in the form of markings of foundries other than the one which cast the piece. It is often theorized that some foundries may have obtained obsolete patterns from other foundries and modified them, sometimes even leaving parts of the original markings intact.
More likely, in most cases, was that actual pans from other foundries were modified, either for use as or from which to create working patterns. We know, from pieces containing ghosts of their own obsolete marks, that several makers updated their existing patterns, so obsolete patterns obviously still held value to them.
Pans from both Ahrens & Arnold and Loth's were evidently made using modified Griswold skillets as patterns. It is unlikely that Griswold would have sold anyone their patterns, from which those other makers' pans appear to have been made. Wapak pieces are often seen which contain Erie ghosts. Look closely at the photo above, and you can see the faint "ERIE" arcing above WAPAK.
Due to the random nature of the manifestation of these "ghosts", there is no documented collectible value to them, other than for what amount various pieces have sold to those who find them a curiosity or an interesting conversation piece.
Here's a little cast iron "spook house".