I'm confused on the notation. Is the following justified or inconsistent?

Solution 1:

The key to any stylistic choice is to remain consistent. As such, as when representing a date range, you should pick a stylistic device and stick to it in all instances—unless there is a very specific reason to deviate in a particular instance.

When it comes to date ranges, most style guides recommend using an en dash rather than hyphen. Also typically, this is closed—there is no space on either side of the dash. (Where the em dash would normally be used in the US, the UK would use an en dash with a space on either side—thereby distinguishing it from an en dash without spaces.)

The Chicago Manual of Style (17th ed.), 6.78, says:

The principal use of the en dash is to connect numbers and, less often, words. With continuing numbers—such as dates, times, and page numbers—it signifies up to and including (or through).

Given that, considering just the date range itself, you should likely pick one of the following styles (which I'm listing in order of what I'd say is most common to least common) and use it consistently.

En dash (closed):

a. 16th–18th century editions of late antique writers
b. Primary sources (16th–19th centuries)

Hyphen:

a. 16th-18th century editions of late antique writers
b. Primary sources (16th-19th centuries)

En dash (open):

a. 16th – 18th century editions of late antique writers
b. Primary sources (16th – 19th centuries)

As for the adjectival modification in the first sentence, conventional styling might say to add a hyphen between the date range and century:

a. 16th–18th-century editions of late antique writers

However, while hyphenation in a compound (barring adverbs ending in -ly or open-compound phrases) is never wrong, it can sometimes look awkward in a way that is more negative than the meaning the hyphenation is trying to clarify. In this case, it's doubtful that anyone would misunderstand the phrase if the hyphenation were removed.

I wouldn't say it's a problem if you don't use a hyphen between the date range and century.

In short, it's up to you how you want to punctuate the date range. The more common styling is with a closed en dash, but that doesn't mean you can't choose something else. However, it is a good idea to remain consistent and not mix your date-range punctuation from one sentence to the next.