If you left before 6 April 2006, you are definitely in AFPS 75.
In 2005-06 there was an Offer To Transfer from AFPS 75 to AFPS 05 (the transfer date for those who opted to transfer was 6 April 2006). Not a lot of people transferred to AFPS 05 but those with service before 18 as an OR or before 21 as an Officer found it attractive, as did those who knew for a certainty that they would have service to age 55 open to them. This latter group were attracted by the idea of leaving at age 54yr 11 months and getting 2 tax free lump sum within just over 10 years.
Like others, my bet is that you stayed in 75 but, to check, ask the paying agent.
You will have a pension in payment so the people to contact are Equiniti Paymaster (who pay your pension) tel. 0345 121 2514 or email
[email protected]. Include your Service and NI numbers in your enquiry.