The term preunit doesn't really apply to RE twins, IMO. The twins were never "unit" designs, like the 250 single was.
RE twins had separate transmissions right up to end of production of the Series II in 1970.
As far as the difference between Meteor and Super Meteor, I believe it was mainly redesigning components like cam profiles, and raising compression to gain a few extra HP.
From what I've read, in the early to mid 50's, the UK suffered from poor quality petrol , so the machines of that era had to keep compression low.
But as better quality fuel became available, motor specs could be increased, thus providing better performance.
I'm sure it's a bit more complicated than that, but the above is probably a fairly accurate statement.
I own some early to late 50's twins and the crankcases are virtually indistinguishable from each other, so I can't comment on the ancestry of the case designs.
In regards to what is more reliable etc.etc. the less stressed design will logically be more long lived and reliable.
So, a low compression 700 or even 500 would be more desirable from a maintenance standpoint.
Since your focus is on the 700 twins, lets' go beyond the Super Meteor and see what happens when a design is pushed to it's limits...
Read about the last hurrah of the 700's, the beautiful but fragile Constellation @approx.50-52 HP : case fretting/flexing caused copious oil leaks (which helped establish the moniker- Royal Oilfield)
And outright blowups were all too common when these machines were pushed to their limits for extended periods of time.
Today, they are few and far between for obvious reasons; and were the impetus for RE to redesign the motor to allow for an increase to a full 750 (736cc actual)
Last edited by oilyamerican; 01/30/20 9:43 pm.