The back boxes with the twin pipes, like you have there, were available in the early 90's. A friend of mine re-shelled a Sprint into a 1500 FWD back then. He had no idea what he was doing but he did have lots of money, so he ended up with lots of expensive bits on it, including the stainless exhaust with the twin pipes. When he cooked the engine due to having an 1850 radiator fitted (I did tell him but he ignored me!), I broke the car for spares and fitted the exhaust to my Sprint. It sounded great, so hopefully yours will too. It might have been available through the DSR, as I have a feeling that's also where he got his vented disc kit from.
I have always wondered why no one makes the twin pipe silencer anymore, as it does look so much better.
The anwer to any question that starts "why don't they....." is always COST! It must have been too expensive to make, to sell for a resonable price compared with the single pipe box. Being more or less straight through you can see inside this one and it has the more expensive baffle design and, of course 2 sets. Since the current Rimmers single pipe Sports back box is over £100 on it's own these days, i'd imagine they'd have to want well over £200 for this one and not many customers would be prepared to pay that when a cheaper alternative was around.
Another possibility is someone realizing they can make and sell a cheaper single pipe box for the same price as the, more expensive to build, twin pipe model, thus increasing profit! If I sound cynical, it's because I am!
Whatever, i'm glad i've got this 90s throwback! It appears to be in excellent shape too!
On noise levels i'm cautiously optimistic, the Rimmers sports system, slightly modified for the shorter Carledo, produces a monitored (at Castle Coombe) 85db at 4500rpm (just over 100mph in real terms) In life, cruising on the motorway at around 3500 rpm, it's not TOO intrusive, even with no back seat or soundproofing and almost no carpet.
In the well soundproofed Sprint and with most of the exhaust (except this back box) in mild steel which is less "boomy" than stainless, it should be quiet enough to hear the stereo at cruising speeds!
If worst comes to worst, i'll have a custom centre box MADE for it that ISN'T straight through, to shut it up a bit more! The engine produces somewhere between 141 and 147 bhp out of the box (depending on source quoted) and was enough to give the almost 2 ton, air conditioned, power steered Omega it came from a good level of performance. I've not altered the map at all (and I don't intend to) but i've fitted a less restrictive air filter and this less restrictive exhaust which might be good for a handful of horses, the ECU can cope with up to 5% power increase, add this to no aircon or PAS to suck power, a taller diff and a kerb weight just over ONE ton and it SHOULD go very well indeed! I can probably afford to sacrifice a couple of horsepower on a quieter exhaust!