Ah. How silly of me. Apologies, and here they are:
I've also tried calculating with sodium and with a charge of 2+... I've been focusing on the 441 peak, because it's so close to my product, but I'm quite curious about the base peak at 921. Analogues of this compound have been known to dimerize in the MS, though.
I have a feeling it's something to do with the ethoxy leaving groups part way through the Pictet-Spengler. But not entirely certain. Seems the NMR is on the lab computer, so I won't be able to get it for a couple of days.