Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.2.18

13.2.2.18

atha yallohamayāḥ paryaṅgyāṇām yathā vai rājño rājāno rājakṛtaḥ sūtagrāmaṇya evaṃ vā ete'śvasya yatparyaṅgyā evamu vā etaddhiraṇyasya yallohaṃ svenaivaināṃstadrūpeṇa samardhayati

And as to why there are copper (knives) for the 'paryaṅgyas,'--even as the non-royal kingmakers, the heralds and headmen, are to the king, so those 'paryaṅgyas' are to the horse; and so, indeed, is this--to wit, copper--to gold: with their own form he thus endows them.