Ah, good catch, thank you, I was going based on the wording of a Times Union article that called it a State historical marker. If the list is indeed complete, please make the note of that invisible, as my note was. When I put a list article up for FL status is when I learned that we dont put statements like "This is a list of..." or "This is a complete list of..." The preferred way of doing an opening paragraph is to actually have a prose that describes and tells about the list rather than being straight forward about it. One I worked on and am proud of is List of incorporated places in New York's Capital District. Some very recently promoted FL's would be the most up-to-date on style preferences used on lists today- such as List of Oregon state symbols which is nearest in type/topic of list to yours that I know of, a search of other list articles of a similar topic and see what they do may be beneficial to this article and the writing of a prose lead. This is really skimpy and just a summary but reading Wikipedia:Featured list criteria could be beneficial if you havent already, or contacting someone who regularly does FL promotion reviews (I can provide you with some names if you want).
Separate from that, perhaps changing the title of the article to expand its scope to include any historical marker would be something you are interested in doing? A separate list article could always be done to cover them. If you're like me and enjoy researching and learning new things, expanding or making a new article for other types of historical markers could be fun and I for one would be very interested in seeing the work and learning more from the hard work that you've shown you do a good job at.