C# - Generate a GUID from an ObjectID

BsonToGuid.cs

using System;
using System.Linq;
using MongoDB.Bson;

namespace Extensions
{
    internal static class BsonToGuid
    {
        internal static Guid AsGuid(this BsonObjectId oid)
        {            
            var bytes = oid.Value.ToByteArray().Concat(new byte[] { 5, 5, 5, 5 }).ToArray();
            Guid gid = new Guid(bytes);
            return gid;
        }
    }
}

Why 5s?