That letter is simply the distribution channel. The number is what's important. If Amazon drops support for the 3500, then it will stop working then.
By the way, the 3500 is a very outdated device, compared to the current lineup. If you replace that stick with a current model, you'd be pleased with the performance. It may be that Amazon did you a favor. Other than making you spend money.
DBDukes Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." Note: I am not a Roku employee.