What you'll see in these early 19th century books are a move to mechanized sewing, especially for the first time end bands, and a transition from leather binding to cloth.
I can't tell from the images, but I'd guess many of these books are quarter-bound or perhaps half-bound in leather, with marbelized paper covers. Binding would have been for the first time under the purview of publishers during this period (hence the matching spine decoration) interested in customers buying sets.
If you've got the time, OGB, I'd be interested in checking out the c18 books and especially the c15 (as that is my period of expertise).
Also, don't listen to booksellers when it comes to support. Call it "parchment", not "vellum" because the latter refers exclusively to calfskin and depending on when and where it was produced it might very well not be vellum at all. Booksellers tend to call every membrane vellum and though I've given up on convincing them I've found myself politely correcting academics who frequently make the same mistake.
Mine are pretty evenly split between full leather (Byron, Grote, Milton, Wordsworth) and half-leather over marbled boards (Austen, Dickens, Disraeli, Scott, Shakespeare). All the edges are marbled as well, which I prefer over gilt (or uncut). They all date from between the 1840's to the 1880's, which seems to roughly correspond with the heyday for those style bindings.
I'll try to snap some pictures of the older books tonight. One of them was rebound in the mid-1800's. Duly noted on the vellum, although it's not really a book, haha. I'm not sure about the format of Cave. It wasn't specified in the listing, and I'm not sure how to tell on my own. Here's the complete listing:
Title: Antiquitates Apostolicae: or, the History of the Lives, Acts and Martyrdoms of the Holy Apostles of our Saviour, and the Two Evangelists, SS. Mark & Luke To which is added: An Inroductory Discourse concerning the Three Great Dispensations of the Church, Patriachal, Mosaical, and Evangelical. Being a Continuation of Antiquitates Christianae, or The Life and Death of the Holy Jesus. The Third Edition.
Author: Cave, William
Publisher: R. Norton
Description: 3rd Edition, printed for R. Royston. 310x215mm, LXVIII+XVII+234+XXXII+335 pages. Original Leather Hardcover with ribbed spine and gilt spine-title. Decorative engravings on front and back covers. Contains several double-page, full-page, and smaller engravings. Cover is very lightly stained and very lightly scratched. Cover edges slightly worn. Spine edges and spine ribs slightly worn. Numerous ink inscriptions on inner front cover, first and third whitepages, as well as first rear whitepage, by previous owner. Also, pencil inscriptions logging past repairs to the book, on first whitepage. All pages lightly yellowing, while engravings are slightly moreso. Almost no age-stains, although there are several tiny brown burnmarks in various locations in the book. Several engravings have been reinforced with a paperboard backing (No damage to the engravings). Several pages repaired. All pages slightly wavy. Page edges red. This rare beautifully illustrated book is in good condition for its age. The book is in : English