Thought I'd provide an update on what has turned out to be quite an eventful investigation into the identity of my S4...
As mentioned at the top of the thread, the car is titled in Australia as 45/8651, and although it has evidently been in Australia for some time, I was told by the PO that it was originally a French delivery car, and it does have a French compliance plate from the original French Lotus distributor, with the unit number imprinted upon it.
What it doesn't have, unfortunately, is the factory chassis plate, so I've spent the last couple of weeks trying to dig up evidence to confirm or contradict its identity.
I have to say a big "thank you" to Andy Graham at Lotus, who has been very prompt and patient in checking numbers and suggesting new places to look for further info.
The summary is that the identity of the body shell is not confirmed, but that the chassis and the engine are both from French delivery cars. The chassis is from 36/6467, a 1966 FHC, and the engine block is from a 1968 DHC.
It appears then that my car is an amalgam of at least three French delivery cars.
I did find a body number on the driver's door, which Andy confirms to be from a DHC of about the right age, but there seems to be no body number on the firewall of the car. I've sanded the paint back in the right area, and Andy confirms that the body number should be in the exposed section. I don't know whether that means it was never there, or has been removed, or whether the firewall has been repaired/replaced at some stage.
Andy suggested that possibly the whole front section of the shell might have been replaced, which could fit with the replacement chassis.
I gather that it's possible that the body number might appear on the transmission tunnel, but at the moment I'm not inclined to take the interior to pieces to find out. It's something I might look at in future, though, especially if I ever repaint the car.
My next steps are going to be to see if anyone in the various Australian Lotus clubs knows anything about the car - I've discovered that it was once registered in NSW, as well as the SA reg I knew about when I bought it - and also to see if the French club can help.
I've been in touch with someone named Damien in France, but it's slow going as I don't speak French and have to get a colleague to translate for me. If there are any French-based forumites who might be able to help out, I'd be grateful!
Enhanced image below of the chassis number, which I found on the right side chassis rail after taking the carbs off, and also a picture of the sanded area of the firewall, sadly with no body number.