It seems like Speakerexchange uses the same .400" coil for their 2123H aftermarket kit as they use in their kit for the 2122H, and there are no coil especially for the 2123H in the MWA catalog either.
JBL claims +/- 2,5mm Xmax for the 2123 but the overhang are 1,8mm so I guess JBL`s number are calculated with a certain percentage (10%?) drop in BL