You are right that those are two different series, but they did change the design of SF Masterworks around number 74-75, and dropped the numbering as well.

I have books from both before and after. The original SF Masterworks series has black covers with a number clearly printed, while the new one has yellow covers.

According to Wikipedia, the yellow version is from a "relaunch" in 2010 onwards, with some of the original numbered books being republished with the new cover as well. Why they felt the need to do that, who knows.

To make it even more confusing, Gollancz appear to have marketed other series with the same name.

Here's a page that shows a bunch of them in both iterations, though without showing the spine:

https://bearalley.blogspot.com/2019/06/sf-masterworks-cover-...

Here's one showing yet another version (the other version of the "yellow" series has yellow spines as well):

https://blackwells.co.uk/bookshop/product/SF-Masterworks-Mas...

I quite like that one. What I don't like is starting to buy a series and then not being able to keep buying the same design... If it's going to keep changing, I'd rather just have them be individual.

Here's someone on Reddit that has two of the SF Masterworks variations (top left, and top right), and Fantasy Masterowkrs (most of the second row from the top)

https://www.reddit.com/r/sciencefiction/comments/q2eiwk/my_h...

From what I can tell they've done yet another variant at some point, but that's titled "The Best of the SF Masterworks" so I guess that can be forgiven for using a different design:

https://www.redlionbooks.co.uk/product/the-word-for-world-is...

(yes, I care more about this than I should; it's made me pretty much stop buying paper books and instead put other things on my shelves)