Thad,
Here is my first pass at Crescent serial numbers by year from digging through period publications/newspapers and serial numbers from existing Crescents. Definitely needs a lot more work and research but had to start somewhere. Curious if anyone else has a list going to compare this to or more info to add to it.
1894 Serial Number Range 70000-121000 (51,000+)
1895 Serial Number Range 121000-178000 (57,000+)
1896 Serial Number Range 178000-248000 (70,000+)
1897 Serial Number Range 248000-331000 (83,000+)
1898 Serial Number Range 331000-431000 (100,000+)
1899 Serial Number Range 431000-561000 (130,000)
1900 Serial Number Range 561000-740000 (179,000)
1901 Serial Number Range 740000-786000 (46,000)
1902 Serial Number Range 786000-?
From Crescent catalogs, Tandem No. 20 was only offered 1897 and 1898. From the sequence above, your 459860 serial number is early 1899 so it could have been left over from 1898 and sold in 1899 even though Crescent did not advertise a tandem for 1899. The beginning and end of these serial number ranges are a little fuzzy so it could also just be a very late 1898 model. Either way, from references found to date, I am confident yours is a 1898 No. 20 Tandem and not a 1897.
The 1894-1899 production quantities above came directly from period Crescent ads. Have not nailed down quantities for pre-1894 so 70000 is a guesstimate based on highest 1893 serial number I have found is 59770.
Jeff