Czy moglibyście wytłumaczyć co to za konstrukcja, proszę?
There be but two ....[/i]
It can be classified as the present subjunctive (formulaic) used with the existential 'there'
as a grammatical subject with 'breadths' being a notional subject here
(not to confuse with an introductory adverb 'there', our 'there' here is lacking stress).
It's rather formal and archaic in style but still very much alive in legal English.
Or, given that this stuff comes from some dusty virtual attic and it's definitely older than my tabby's piss in the snow (just in case, this is my saying, all rights reserved:), it may just as well be a Middle English Chaucer's way of using 'be' instead of 'are', as the plural of the third person present indicative.