@chris87 The iron pipe, rebar, whatever was placed in the ground is the main body of the monument,?ÿ A cap is placed on it for the purpose of facilitating markings.?ÿ The cap's absence does not negate the monument's status in any way.?ÿ As long as the pipe, rebar, whatever still exists in its originally set position, the corner remains monumented, and unless you have good reason to believe it is anything other than the pipe, bar, whatever driven into the ground by the original surveyor, it establishes and fixes the corner on the ground at that position.
If the cap was plastic, the expected lifespan is anywhere from a year to several decades, depending upon its exposure to sun & heat, or until a brush fire occurs and melts it off.?ÿ If it was aluminum, it will hold up better to sun and heat, but may still melt with a hot fire.?ÿ Many aluminum caps are also held on with a plastic sleeve in the neck, so a fire that's not hot enough to melt the aluminum would melt that sleeve, leaving the cap just sitting loosely on the pipe - easy to knock off by someone digging, mowing, or just walking by.?ÿ Lots of other ways a cap can get knocked off.
In evaluating whether it is original, if the type and character, and in some cases, the manner in which it was set appears to be the same as the other originals (except for presence of a cap), then it is most likely original.?ÿ If it's in the correct position as related to other monuments, it's immaterial.?ÿ Even if it's not an original, you should be accepting it as properly marking the corner.?ÿ Add your own cap or tag and move on.
If you have reason to believe it is not an original (other than the missing cap), you have no record of its placement, and the difference in position between where you found it and where you expected to find it is considerably greater than can be explained by reasonable differences in measurement between surveyors (take into account the type of equipment and probable methods that would have been used to set it), then you might have good reason to reject it, but I always start with the assumption that I should accept a found monument until I have good reason to reject it, original or otherwise.
On your map, you don't make note of "original" or "not original", you note the type/character/condition of what you found and either graphically or by note indicate whether you accepted & used or rejected the monument.