This is for the whole phrase "a boca y escoda", which, as Taña has correctly suggested (I think), is being used in the sense of "a boca de escoda" (it may be a simple mistake, misremembering the phrase, of perhaps it's a variant).

"Escoda" is a stonemason's hammer, which is rather like a chisel with a handle:
http://www.artifexbalear.org/escodra.htm

So it's used for cutting stone. It's no use for hitting a chisel, and there's no reason to think a chisel is involved here.

"a boca de escoda
Locución adverbial que designa una técnica de trabajo propia de los canteros, para cuya comprensión traigo a colación estas dos definiciones del DRAE correspondientes, respectivamente, a boca 'en algunas herramientas de percusión, como martillo, maceta, martellina, etc., cada una de las caras destinadas a golpear' (s.v., 3.a ac.) y a escoda 'instrumento de hierro, a manera de martillo, con corte en ambos lados, enastado en un mango, para labrar piedras y picar paredes'."
http://ifc.dpz.es/recursos/publicaciones/09/23/24frago.pdf

So it means with the cutting edge of a stonemason's hammer. The corresponding phrase in English is "hammer-dressed":

"Hammer-dressed
a. 1. Having the surface roughly shaped or faced with the stonecutter's hammer; - said of building stone."
http://www.thefreedictionary.com/Hammer-dressed

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Note added at 7 hrs (2015-11-13 22:59:46 GMT)
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In fact, looking more closely at the ST, this covers the whole of "labrados a boca y escoda".