The model number you're seeing, MNAF2LL/A, is actually a part number, which is different from the model number. Apple uses part numbers like MNAF2LL/A to identify devices for sales and service purposes. The part number can tell you about the specific version of the device, including storage capacity, color, and the regions it's intended for.
For the iPhone 7, the model number does start with an 'A'. In your case, the model number is https://www.bing.com/search?form=SKPBOT&q=A1660, which is associated with the iPhone 7 (32GB) version that was released in 2016 https://www.sellyourmac.com/mac-product-guides/iphone/mnaf2ll-a-iphone-7-32gb.html. This model number is used to identify the device's hardware specifications and capabilities. It's common for Apple products to have both a model number and a part number, and they serve different purposes. If you need more detailed information about your iPhone, you can usually find it in the 'Settings' under 'General' > 'About' or on the back of the device.