It's time to make a few notes, some observations, some speculation, some assumptions based on the work was by Cosworth Engineering themselves. The car was built by Geoff Mansell, aircraft engineer, not working as such. The first thing to debunk is that Cosworth never sold direct, recently I "met" another owner (son of) who also purchased direct from Cosworth back in those days.

https://www.caterhamlotus7.club/forums/topic/276429-bdr-how-do-i-know-what-my-max-rpm-is/?do=findComment&comment=2620460

Pictures of the inside underneath of the engine are here:

https://www.caterhamlotus7.club/forums/topic/276429-bdr-how-do-i-know-what-my-max-rpm-is/?do=findComment&comment=2620457

So my post wondering what my max revs might be, was a way of exploring what about my engine is non standard and for the first time I have some clues.

All work by Cosworth Engineering

The crankshaft has had balancing work - Balanced Crankshaft
The Conrods have had balancing work - balancing usually involves all conrods weighing the same to within one gramme.- Balanced Conrods
ARP Conrod bolts 
Strikes me if the Conrods have been balanced so must the Pistons unless they come that way. 
We have non standard Two Bolt Main Caps (not cross bolted). Google indicated they look billeted (cut from a solid lump of metal (steel?)). Billet Main Caps
Strikes me with both a balanced Crankshaft and (possibly) Billet Main Caps Cosworth would have line bored all the bores to match - be nice if this can be seen. Line Bored
Now then, the pistons. Geoff told me the car has xxxxx pistons. My memory thought this was steel, but that's not possible. However, all the above work is consistent with their being Forged Pistons. In fact all that work would not be worth it without pistons to match. - Forged Pistons

I find it notable that Geoff did not seek to increase the 170BHP of the engine. The Camshafts are standard BD3 and the head is apparently un modified - though without removing the head I can't see if there was any porting. It seems smooth power delivery, throttle response, longevity, drivability and fuel efficiency may have been of more interest, as well as max power rather than round town. Perhaps he enjoyed hill climbs within a certain class for example.

Think I will embolden the bits I think are validated/real - even if slightly enhanced by me and italicised bold if supposed, not authenticated

So what I have is one seriously toughened and improved bottom end Ford Cosworth BDA (derivative R) 711m block engine.

and come to think of it that should be good for more than 7,000 rpm and I never hesitate to let it go there, but since the tacho stops there so do I - and I suppose if max power is at 6,250 there's no point anyway.