A 1961 model year should have come with the B16 and 4.56 rear end. But the cars are getting a little old now (ahem) and there has been ample time over the last 5 decades for people to swap parts around. And the supply of B18 and B20 motors is vast compared to the supply of B16 motors. And the parts supply situation is similarly more difficult for a B16.

Already mentioned, but with some visual aids here. The B4/14/16 valve cover, easily seen in even a tiny blurry picture in an ad (if they have a pic of the motor) is rectangular, square-ish curved edges, with some creases in it. It's held on by two nuts through the top side of the cover.



The B18/20 valve cover is trapezoidial, has rounded edges, and no creases or grooves in it. It's held on by 4 screws on its lower edge:




Additionally, a '61 PV would also have had a 6 volt electrical system. But it's not that hard to convert a 6V PV to 12V, mostly because the electrical system is so simple.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)