Difference of "I am just an ABC" vs "I am but a XYZ"

As far as I (non-native speaker) can tell, these two sentences have the same meaning:

I'm just a humble merchant

I'm but a humble merchant

However I wonder if there is some subtle difference between those two variants I am missing. From the contexts I found the variants in, the one using but seems to be a bit more anachronistic and - when used in a modern setting - seems to have a hint of irony/sarcasm to it? Is there any modern context in which one would use but instead of just for other reasons (maybe indicate belonging to a special social group)?

But and just as an adverb both mean merely. Although but has negative connotations: but is synonymous with nothing but/nobbut and is interchangeable.

I'm but a humble merchant
I'm nothing but/nobbut a merchant

Consider the sentence "I don't have but one pen". The meaning of the sentence remains unchanged with or without the don't. Just is more emphatic than but is dismissive.