As asked previously, to verify whether your first 18 volumes are rebound reprints, please show us pictures of the following:
June 1899 map supplement upper right corner to include all the margin as well as December 1899 map supplement title to include the margin below the scale.
The word "REPRINT" will be visible if indeed these are rebound reprints (with covers and ads stripped out).
(note: Not all loose map supplements in the reprints had the "REPRINT" stamp on them and none of the "larger than the printed page" foldouts that were bound within the reprints (individual issues or bound volumes) had this word printed on them.)
As I stated earlier to you on eBay, you'd be surprised how often this happens - either because someone wanted to make others believe they had all originals or (as I prefer to believe) some collector wanted their entire set to appear homogeneous and this was the standard method of binding NGM's back in the day.
The even larger loose map sheet for the December 1899 issue is titled, "Map of the Seat of War in Africa."
The picture needs to show the margin below the scale, which is located below the Title located in the lower right quadrant of the map.
Please stay with me on this, the details are important, "larger than the printed page" foldout maps bound within the issues DID NOT HAVE the word "REPRINT" stamped on them.
Only the large loose map sheets (usually enclosed in pockets in the back of bound volumes) - and then, not all of them had it.
If you do not have these loose map supplements, your set is not complete.
6034 is not the loose map for the June 1899 issue.
The title of the large loose map is, "Theater of Military Operations in Luzon, 1899."
1) only "loose" maps between 1888 and 1907 "could" have the word "REPRINT" on them. Every other supplement "after" Dec 1907 will be original, e.g. the reprints were only for the years 1888 through 1907.
2) Not all "loose" reprint maps have the word "REPRINT" on them - a traditional NGS nonstandard method of doing things, even in 1964...
3) no bound reprint supplement "larger then the printed page" foldout has the word "REPRINT" on it - so if the supplement had been removed during the rebinding process and set aside as a loose supplement - it would not have the word "REPRINT."
The attachment is an old summary. Since I wrote this, my collection has become nearly complete which has only confirmed the data I represented in the attached summary.