This question comes up relatively often (and for good reason), and we thought it would be nice to have an explainer on the site to refer back to, so here it is:
Deluxe Editions
The first and longest-used application of Roman Numerals in Centipede Press has been to denote Deluxe Editions, these being editions of generally un-fixed quantities in the 10-30 range (unlike a 26-letter scheme, for example) that are upgraded from the Standard Edition in any number of different ways. These can include combinations of better paper stock, larger size, and upgraded materials (usually leather in some part or the whole), and a traycase. They were much more common at the beginning of the Press and especially during the run as Millipede Press (where I think almost every book had a Deluxe counterpart).
"Standard vs. Deluxe Editions" is a post unto itself, for the future sometime. In terms of visibility across varieties of collectors, the most famous Deluxe Edition is probably the same as the most famous Standard Edition: Salem's Lot. Long hailed as one of the best treatments of any King novel,
and a grail to many King collectors, though at different orders of magnitude: a Standard Edition (left, not to be confused with the less-expensive and unsigned Gift Edition) is in the $3k+ range and a Deluxe Edition (right) is in the $15k+ range.
CBV on Salem's Lot Standard Edition:
CBV on Salem's Lot Deluxe Edition:
Most Roman Numerals are black. However, as we have grown to love about Centipede Press, there is no rule that isn't broken at some point in its multi-decade history, and this is no exception. In fact, the Roman Numeral Salem's Lot was the first with two different colors of Roman Numeral - 15 black (public sale) and 10 red (presumably all or most to King for distribution. This was back in the very first few books where the limitation pages were hand-numbered and not printed (that changed shortly thereafter). I'm certain there are other cases. I could see red Roman Numerals as the designation for contributor's copies (effectively PC copies) for a Deluxe Edition. But there are probably other reasons, too.
Standard Edition Contributor's Copies
Speaking of contributor's copies: this is the other use for the Roman Numeral designation! These are essentially the Centipede Press version of PC copies, in that they are for contributors and replacements and not generally for sale. Instead of a single designation ("PC") across all copies, copies are numbered with Roman Numerals from I-XX (or whatever). This is a much newer phenomenon than the Deluxe designation, which goes back to the beginning of the Press. But still, it's been going on since at least 2015, maybe even earlier in select cases. Additionally, they are made for most titles, unlike Deluxe Editions, and that of course heavily contributes to their presence in the marketplace. At this point any random Roman Numeral designated book is more likely to be a contributor copy (I can't even bring myself to refer to them as PC copies! It certainly doesn't affect the secondary market price like most other presses) than a Deluxe Edition, so obviously that's going to raise a lot of questions.
How to Tell the Difference
If you've found a book and you're not sure, the first thing you can do is look at the date. By 2015 Deluxe Editions had grown extremely rare and the use of the contributor's copy had picked up. A Roman Numeral designation pre-2014 would most likely be a Deluxe Edition and a Roman Numeral designation after 2015 could still be a Deluxe Edition but is much more likely to be a contributor's copy.
The tell, though - always - is the construction of the book. Contributor's copies are identical to the Standard Edition of the book, in every way save the designation run. Deluxe Editions have extra design features, better materials, etc. Until the Deluxe Editions of the Elric books, they were always unjacketed, but that's not a good indicator on its own because prior to 2012 most books were unjacketed. At this point CBV has built up a pretty good reservoir of pics so you can get visual confirmation for many of them (and growing, with a complete list coming soon) by looking up the book in the bibliography.
Well, I hope this helps clear up some of the confusion! There are almost certainly exceptions out there but I think this delineates the two general uses. Keep in mind that there are lots of variants early on in the press, and this by no means excludes the Deluxe Editions. There are also a few one-off editions out there done at the level of a Deluxe Editions (those are rare birds indeed!). If I forgot something dumb or if things change, I'll edit this post if possible. [Sometimes the caveats at the end of a technical CP post feel like side effect warnings in drug commercials: "maybe I'm wrong", "maybe it's a different variant", "maybe it's a one-off use", "don't listen to me", "well maybe do listen to me but be skeptical about it", etc.]
Edit 1: The two photos of Salem's Lot are from the respective CBV pages. The cover image of "This is copy number XXX" is from a photograph of the deluxe Elric of Melnibone by Yegor Malinovskii.