Indeed, the 321T aircraft made frequent rounds JFK-BOS between Christmas and 1/4 a bunch (for what I assume was a reshuffling of capacity during the holidays). It was very nice to have a full flat reverse herringbone seat for the 30 min flight, although I found it amusing overhearing the conversations of gate agents in BOS who were quite puzzled and unfamiliar with the cabin configuration.

I've seen them on other randomly scheduled BOS-JFK services since then and, I too, am booked on one again in early March BOS-JFK but unsure the rationale for these BOS-JFK flights. I have to assume the aircraft was going to be idle in JFK for a few hours and it made more sense than tying up another 738 or 319 on the route.