A Monsignor in my diocese - here in Scotland - uses a Mantelletta and a Biretta with a Violet Pom. He isn't the highest rank of Monsignor, but continues to wear it in regards to Tradition. The Violet Pom is because he is also a Canon of the Cathedral Chapter. He's a great guy, and always celebrates Mass with such piety and dignity; with respect for tradition and reverence for Our Lord. Deo Gratias!
Perhaps the mantelletta is part of the vesture of being a canon of the cathedral chapter.
If the pom is violet, then he is a Papal Chaplain - the lowest rank. Even before Vat 2, this grade of the Monsignori wasn't entitled to that garment, but wore a violet mantellone. The mantelletta was for the highers grades: Domestic Prelate, now a Prelate of Honor and the Protonotary. These retained the violet cassock and sash, but the mantelletta was abolished. I was among the last to be named a Protonotary, so I had someone make a mantelletta for me, which I do wear, since I work out of the Nunicature - after all, that makes me a member of the Papal Household. I'm not overly concerned with what the present pope thinks about it. I worked doing a VERY nasty job for the last 25 years for HMC, and I have earned it!