When I spoke to RSA they told me there claim history letter will show the actual full amount of years that you have not had a claim with them, when I asked about mine earlier this to year, I was told currently 8 years
I’ve had a quick look for the letter but its buried somewhere but I’m 90% sure its 5 years NCB unless its changed since 2017.
The letter is not even a NCB letter it is in fact a “Good driver” letter that’s recognised by insurers as 5 years NCB and AFAIK is why not all insurers except it but could be wrong unless something has change or perhaps some insurers take your word it worth more than 5 years but wouldn’t think so.
Although I have that letter here somewhere I have published it on this very forum with my personal information removed in the passed 5 or 6 years so possible for it to be dug up.