Is the expression "topping it the ______" really used anywhere?

I've often read books where English men made statements about others "topping it the knob" or "topping it the gentleman," which I took to mean over playing a role or attitude.

Is this a legitimately used idiom, or is it archaic, or even possibly fabricated?

I don't recognise this idiomatic usage, but there are a dozen or so instances of top/topping it the knob/nob in Google Books - so in a limited sense, the expression does exist. But that link is typical for all verb forms and spellings - they're mostly novelist Patrick O’Brian, from 15 years ago or less.

I suspect this use of to top [it the something] relates to OED's sense 15:

Theatr. to top one's part, to play one's part to its utmost possibilities or to perfection; to transcend the character assigned to one; transf. to sustain (a character) with success. to top the officer (Naut.)

Thus, "to top [it] the xxx" can be seen as being related to:

"Don't come the high and mighty"

"He's always playing the fool"

"I'll just act the innocent"

I also suspect "knob" is always incorrect here, since I'm pretty sure the nob being referenced is noun - Chiefly British Slang. a person of wealth or social importance. Since I find no evidence of "top it the [noun]" being used earlier than a couple of decades ago, I doubt it's "archaic". It's either a (deliberately or accidentally) garbled version of "top the [noun]", or an obscure dialectal usage.

I’m also familiar with the phrase from the Aubrey/Maturin series. It is apparently used as a pejorative reference about someone “topping the nob “ by putting on airs, acting above one’s station, a common person acting like a “nob” or nobility.