I think there's a certain amount of confusion because of the '850' label; in the UK most R4 owners will think '845' (Billancourt), whereas in Spain, for example, '852' (Cléon) might be what websites are referring to. It's certainly easy to end up with the wrong part - look at this, for example:
Hi Andy
Thanks for your comments. Appreciated.
Guy near me in MK purchased the Spanish F4 that was on Ebay some weeks ago.
He is having overheating problems and I have a spare 845 rad hence the question.
I have appointed myself his research assistant!!
The refereences you give above will be very useful. Thanks.