Which preposition to use with armholes

of Occurrences 11%

He sat down at the table with his back to the light, and carefully examined the thing back and front, while the detective, glass in hand, cigar in lips, and thumb in the armhole of his waistcoat, watched him appreciatively and inquisitively.

with Occurrences 1%

You could have got it just as easily as not!" He laughed and thrust his thumbs in his waistcoat armholes with the gesture she disliked.

Which preposition to use with  armholes