This comment was found on the listing for a 20B on ebay recently.......(item
# 110190900577).
Who knows how to find out if you HAVE these 'updated style sideplates-thick
dowel pin area' items without taking it all apart?

"Perfect for your RX7 FC3S FB SA or FD3S swap! Here I have a tested Jspec (
low miles), only 34,000, 3 Rotor twin turbo 20B. Comes with an uncut harness
and ecu. Tested, over 100psi on all 3 rotor housings. This is a complete
longblock.  This motor has the updated style sideplates ( THICK DOWEL PIN
AREA) that dont crack like the early type...major plus if you are planning
to make big HP with a HUGE single turbo.  Buyer pays shipping and I can ship
just about any place where a Freight Co. can deliver. I accept confirmed
shipping address paypal payment and US postal money orders only..maybe a
wire transfer if you are overseas..  Anyway, this is a rare engine and they
are hard to find! Payment in my hand or in my account within 5 days or less
of auction close"

Marc Wiese

You can tell by looking at the third gen housings.
New post 1992 3rd gen housing on the left. Picture by Rino LaComb