It's been several years since I went through there; if there was any signage it would be the "SD Secondary" style of a small black and white rectangle with the number on it. The thing with sparsely signed SD routes is that they are usually very obviously marked on maps (see
SD 251 for an example) or otherwise known as state highways, but 203 is so short I'm not sure how many people know about it other than roadgeeks and people who bought the SD DeLorme. From what I remember, SD 258 had the "SD Secondary" signs but I didn't see any for 203.
Probably didn't help that I'm pretty sure I clinched it in the dark.