Mullard 12AX7 I61 and I63 differences

[Cheech]

I have two tubes I have bought in various lots that I am positive they are mullards. However one is lacking date and the "I" code. The other just has a date code.

The one with just date code says B4H4. This has copper control rods. However it has a different color top mica. One is near white and the other is shiney. It also looks like it has a extra hole on the top mica.

Are there ways to tell the difference? I believe both to be I63 but I wanted to make sure ones not a I61.

Will

[Cheech]

Here is a picture of one in question. Can you guys identify its time frame. The I61 from 1959 I have has a different top mica. The I63 I have from 64 and 65 have different top mica than the earlier I61. I have another that has a top mica similar to this one but its a tad thinner. Could this mullard be one of the 10M series? Sounds compressed but its very alive and I seem to like it a lot.

[Cheech]

Actually not sure this is a mullard anymore. I am leaning more twards a Philips Heerlen Holland made. What you guys think

[Cheech]

Ya T confirmed that this is a Philips. Sounds very harmonic to me. However it could just have microphonics. I have another philips thats same structure that I found after I posted this. However it rates less in micromhos than this one. Maybe 200 less on each triode.

Good tubes none the less.

[Melodyman]

I have 3 NOS Mullard 12ax7's that say Daystrom on them but have no i61 or i63 etched in the galss. How do I tell which they are? Is there any other identifier that tells us which they are?
Thanx!