I believe that the Studio 4 was fairly early in TDLs history, after the transition from the IMF like monsters. The later models all used a pair of conventional circular woofers in MTM
I think you're referring to the RTL 3 - Which appears to be the most widely sold late TDL Electronics design (probably accounts for half of the proper, pre - Richer-Sounds TDLs you see on fleabay), but this was by no means the only model they were selling at this stage. The RTL range were at a lower price point, & were not a transmission line design, despite the name, and ran in parallel with the Monitor and Studio ranges. Some had dual bass drivers, as you described (RTL 3 and 4, the later T-Line 3) Some didn't. (RTL 1 & 2, and then the T-Line 2 had a single LF driver below the HF)
The Studio 3 also had this arrangement (2 LF drivers above and below the HF) but the Studio 1 & 2 (2 drivers, LF above HF) and 4 (3-way featured in this thread) did not. The 4-way Monitor, Studio 2 and 3 were eventually dropped. The Studio 1 went though quite a few revisions, eventually becoming the 1m; the very small floor standing transmission line Studio 0.5 was briefly available - and then eventually the Studio 0.75m, 1m, 4 and the RSTLm continued in parallel with the RTL and then the later T-Line range until the rights were sold to Richer sounds, who scrapped the whole range and put the TDL badge (dropping the "Electronics" part of the moniker) onto some budget nonsense of their own design. (Studio 5 & 10, Saturn, Nucleus etc)
In addition, there was a parallel range of TDL Monitors distributed in Europe by A.O.S.- mostly in kit form. Some were a 4 way, like the Monitor here, and some were a three way - similar to the Studio 4, but with the mid above the HF (known as the monitor compact) The distributer's product line started to diverge from the UK line when the Studio 4 was released, and they eventually started selling their own transmission line monitors with ATC & Scanspeak drivers.
Confusingly, the name "Monitor Compact" had been used by TDL a few years earlier in the UK for a bookshelf sized speaker.