I most commonly see the CP05A for a military retiree that has qualified for VA disability benefits retroactively. Is this your case? You did state the notice is asking for "statement of benefits on retirement income". At 40 years of age it is perfectly possible for you to retire with 20-22 years of military service and qualify for the retirement pay. I myself retired at the age of 38 with a 10% service connected disability that exempted 10% of my retirement pay from taxation.
