Later:
Yes…that plastic guide went in from the outside end of the rack cylinder, and when carefully driven with a 15mm socket (slightly smaller than the smallest I.D. of the slightly tapered cylinder wall), the flange snapped into place in its groove:
I then dropped the flat plastic spacer ring into the other end of the cylinder, followed by the seal…allowing it to rest above the spacer and orienting it with a pick from the other end - to make sure the “lip” side faced towards the inside. To drive the seal, I first began with an oversized driver…consisting of a 26mm socket (on a long socket extension) - reversed to present a broader “shoulder,” and with this dropped the seal to be flush with its metal locating flange. I then moved to a 22mm socket and, peering down the tube of the cylinder with a good headlamp to assure concentricity, drove the seal down further until it bottomed out on the plastic spacer.
When I use sockets as seal drivers, I like to use them upside down - for a greater contact area pushing the seal. Do note, from the photo…that there is still a very slight amount of taper on the end of the socket’s backside. Despite this, I still find a backwards socket orientation preferable to forwards for seal driving. But YMMV.
