AtD translation: picked up a minor angel

[Mike Jing]

P725.20-29   UP HERE IN NORTH ITALY, as in France one might buy ordinary
village wine hoping to find a few cases of overrun from a great vineyard
nearby, Vibe’s theory was to buy all the school-of-Squarciones he could put
his hands on in the hopes that someplace in there might be an unattributed
Mantegna somebody had overlooked. It was the current fashion to disrespect
the painting skills of the famed Paduan collector and impresario himself,
so any actual Squarciones kicking around, including embroideries and
tapestries (for he had begun his working life as a tailor), would be going
for a song. In fact, Scarsdale had already picked up a minor angel just by
singing “On the Banks of the Wabash, Far Away” to a sacristan who might
have been insane.

What does "a minor angel" refer to here exactly?