My first guess would have been that he could be talking about the invention of the nick.

But this "imitated from Gothic forms" clearly indicates that what is being referred to is the letterform. Hmm. Printing Types by Updike is said to devote considerable space to Johann Froben, whose chief claim to fame appears to have been scooping the Elzevirs with a Greek New Testament by underhanded means.

Updike says that the printers of Basle generally used a "squarish" style of Roman type...

Maybe he means with a prominent back-slanting axis? The letters I found on a Google search do look like they have heavy northwest-to-southeast bits (see below, esp. p, a, m, d).

"the peculiar mannerism--probably imitated from Gothic forms--mentioned above,"
Hmm – what is «mentioned above»? Anything that might hint at an answer?

I have a full-page specimen of Johannes Froben's «Basel Type» (of 1521) in a book, let me know if a scan would be of interest.

For some reason I couldn't find the previous mention.
Don't go out of your way for the scan, but thanks for the offer.

Uh, does that mean you'd like one or not? :)

That means no but thanks anyway.

Craig, I would interpret ‘cut on their backs’ and ‘imitated from Gothic forms’ to refer to the flat diagonal cut on the arches of the m and r in your image.