Virtually all retracting-nib "safety" pens trace their origins to the 1890s, when all the key features used in following decades were patented.  The Moore safety was no exception, based on patents of 1893 and 1896 and put into production in 1899.  The Moore was distinctive in using a straight-line mechanism, in which a sleeve sliding over the lower barrel was pushed and pulled to extend and retract the nib.  With the nib retracted, the cap could be screwed down to seal the barrel tight -- so tight, that Moore pens were shipped from the factory already filled with ink as proof of their other name: "Non-Leakable".

Moore pens were made in Boston by the American Fountain Pen Company, to which the pen's inventor, Morris Moore, had sold his patents (and his name).  In 1917, American was reorganized as the Moore Pen Company.  The basic Moore safety design changed very little during the thirty years it remained in production, especially after the phaseout of the original, pre-1902 design shown above, with its small, flush-fitting cap and longer, slotted barrel sleeve. 

At some point a central interior post was added to the cap, to prevent the nib from being damaged by inadvertent replacement of the cap before retracting the nib. The date of the post's introduction has widely been placed in the late 'teens, though it was likely significantly earlier.

The factory cutaway above and the disassembled pen below nicely illustrate the elegant simplicity of the design.  There are only nine parts (cap, barrel, sleeve, end plug, feed & shaft, nib, nib retaining collar, seal, threaded seal retaining washer), which can easily be taken apart for servicing.  There is little complex machining and a minimum of fine-tolerance fitting.  Note that the seal consists of two parts: a thick rubber washer, placed closest to the ink chamber, and cork.

Moore safeties were made in a wide range of sizes, including a vest-pocket sized Midget and a giant, comparable in dimensions to Parker's Black Giant and the Waterman 20.  The vast majority of Moores were of relatively modest size, however, carrying #2 or #3 nibs.  Trim options were many, so one will find Moores with all sorts of smooth and patterned gold filled bands.  

Full and half-overlay pens were also made, and are highly desirable.  The overlays themselves mostly appear to have been made by Heath, the New Jersey firm which also supplied metal overlays to Parker, Waterman, Conklin, and other prominent penmakers.

Some Moore safeties were made in mottled red and black hard rubber, but they are not at all common. Although Moore began offering lever-fillers in 1918, the Non-Leakable remained in production to the very end of the 1920s, making it one of the most successful fountain pen designs of all time.  Although it was not copied in the United States, some of Montblanc's earliest (and rarest) safeties were closely modeled on the Moore.  This is no accident, as one of Montblanc's early partners had previously been an important player in Boston-area pen manufacture.

The pen  above is a late "Improved" or "Banker" model, whose cap can screw on the barrel (if the nib is retracted) or slip on (if the nib has been left extended).  The oval cutouts in the large barrel sleeve permit the barrel to be gripped to screw the cap on and off.  Other less common models include the "Twistout", in which the barrel sleeve is twisted to extend the nib; a sleeve-filler, which closely resembles a normal safety; the "Ink-Tab", which carries its own ink pellets; and the Non-Leakable stylographic.

Moore safeties can still be safely used, provided their seals have been replaced.  They are filled and used like other retracting-nib safeties, and like other such pens, they can tolerate thick, fast-drying inks (such as India ink) that would quickly clog conventional pens.  When replacing (and removing) the cap, remember to grip the barrel and not the sleeve, lest the nib accidentally be extended, risking damage.  Moore safety nibs are often very flexible, but look carefully at the nib of any unrestored pen obtained from a nonspecialist.  Flex nibs may develop cracks if abused, and careless handling has damaged the nib tips of many a safety pen over the years.

Link to Moores for sale