State pension query

I have 49 years when according to my NI summary I have paid full contributions with a gap in year 1977-1978 which I was unable to make up before claiming my state pension ,I am now £659 a year worse off as I do not qualify for the full amount because of the gap. I have phoned to ask for an explanation 3 times with 3 different answers. I decided in March of this year to write a letter requesting an explanation in the hope that if it was in writing it would at least make sense!. Unfortunately I'm still waiting for reply and am doubtful if I'll ever get one.?

Hi mikky,
The team who will deal with your enquiry are currently dealing with work received on 15/08/2023.
A reply will be issued, but all work is actioned in data order.
Thank you.