The purpose of the HMACSHA512.ProduceLegacyHmacValues Boolean property is to enable net_v20SP1_long applications to interact with dnprdnlong applications. When you set this property to true, the System.Security.Cryptography.HMACSHA512 object produces values that match the values produced by the dnprdnlong. You should set this property only once after you create your HMAC object. You will need to reset your key afterwards, as shown in the following example. 

public static void Test()
    {
    HMACSHA512 hmac = new HMACSHA512();
    hmac.ProduceLegacyHmacValues = true;
    hmac.Key = // ...Get the HMAC key.
    // ...
    // Use the HMAC algorithm.
    // ...
    }