When Mordaunt Short stopped producing the 8 inch chassis bass/mid unit they'd used in many of their models and which Naim used, with their own modifications, in the SBL, Naim designed an imroved version to replace it and enable them to keep producing the SBL.
The new version had a nice cast alloy chassis, which didn't need as much damping treatment as the old pressed steel chassis MS sourced unit, and it had a 2 piece phase plug in the centre, the inner part of which was held by a brass bolt though the rear of the magnet to cure the old problem of the original single piece phase plug falling out (it was only held by a blob of araldite).
The 2 piece phase plug consisted of the inner section bolted in place with an outer section held in place, as mentiond above, with a small diy style wall plug. The joint between the 2 sections was around the front of the voice coil former, just about level with the centre of the cone.
Naim used to sell a "repair kit" containing 2 wall plugs and 2 rings of double sided sticky tape for about £30!
The new driver was considered such an improvement over the old that it was offered as a dealer fit upgrade to previous models at a cost of, if I remember correctly, £300 a pair (in 1996).
With the new driver available Naim introduced 2 new speakers, the entry level Intro and the visually identical Credo.
In 1997 the SBL cost £1830 a pair (plus £140 for the passive crossovers!), whilst the Intro & Credo were £660 & £1060 respectively.
The Intro & Credo abandoned the steel sub frame that the SBL was assembled onto in favour of more conventional looking boxes which were easier to move around (and no doubt manufacture) but which still used a simplified form of "separate box" operation with the SBLs gaskets replaced by small spacers which kept the 2 boxes separated by a small air gap.
I gather that the Credo was more carefully built than the Intro, with selected components used, whilst the best and most closely matched drive units were reserved for the SBLs.
In due course the new drive unit was also used in the Allae and later in the SBL replacement, the SL2.