It should be CV4015. However, the Common Valve number, CV4015, is not at all important. It is simply an old British military inventory number for this kind of tube. And in fact, the British military considers the CV4015 and CV131 to be identical. CV4015 is the older number and CV13 is the newer.
The number that matters is M8161 (or 6065). This tube number indicates that it was considered to be somewhat better than the consumer grade EF92. Here, "better" typically means a longer lifetime, more resistance to shock and vibration and so forth. That is, it was designed to be used in more severe environments, such as a military setting or on a factor floor. And this does not mean it will sound better. After all, these pentodes were never designed for use in audio circuits.
IMO, when comparing tubes as above, it is most important to know in which factory and in what year the tubes were manufactured. Without these facts, the above comparison is relatively meaningless. Again, in my opinion.