( IE: 108 in the 4315 and 108H in the L250 - neither were listed in catalogs )
Just to clarify, the 4315 used the 2108 (AlNiCo) or 2108H (SFG ferrite) which was the pro version of the consumer 112A (AlNiCo) or 112H (SFG ferrite) (L212). The L250 and 250Ti used the 108H which was the consumer version of the pro 2118H.
When the 2231H was repainted black, it got a new coil + glue and became the 2235H. They are VERY close in performance and if needed to be replaced, should be done in pairs.
http://audioheritage.org/vbulletin/showpost.php?p=46738

Mark Gander did the 136/2231 and Greg Timbers did the 2235. The 136/2231 was a severely overdamped design with very high compliance as was its 12-inch sibling the 124/2203 (extremely powerful motors, loose suspensions). These older drivers were completely dominated by the enclosures they were typically used in (reference their Vas values).