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How does a lower current for the heater at the same voltage affect GM?

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I have many 1193/CV6/6S5S tubes. They all have a 300mA heater draw and measure from 2700 to 3300 for mutual conductance with a minim acceptable of 1900. The plate current varies on the tube and is higher than the D1148 Hytron but that is not the question.
I have over 100 E 1148 Hytron tubes that were made in 1944 under contact to the US government for the military. They have a heater draw of 200mA. The spec sheet shows they would measure the same as a 7193 but they do not.
They sound excellent and work fine but I am trying to figure out why they by design or because of a lower heater current, measure typically 2000 to 2300 in Mutual conductance/GM and a lower plate current. Gain seems to be the same but for some reason it seems that Hytron varied from the specs in a lower measuring tube. I would put the minimum at 1300 to 1400 GM vs the 1900 (on a Hickok) as the minimum.
I have two 539C calibrated testers that I can test current draw on and a couple of TV7 calibrated (Dan Nelson)testers and they all match up.
These are all NOS in original military boxes and never used. All the 7193 types use 6.3 volts for the heater.
Any ideas?

iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.

Edits: 10/22/23 10/22/23 10/22/23 10/22/23

Follow Ups:

RE: How does a lower current for the heater at the same voltage affect GM? - Triode_Kingdom 21:42:00 10/22/23 (2)
I read through your post several times, but it's still not clear to me exactly what you're asking. It doesn't seem to be the question in the subject line. Are you asking if the lower-than-expected Gm is the result of lower filament power? Or maybe the different filament structure (or some combination of the two)? Or are you asking why the tubes don't agree with their data sheet? And what's the basis for comparing this tube to the 1193? Is that tube supposed to be a substitute?
- Yes, the E1148 is a direct replacement for the 1193/CV6 tube. . . - John PA 22:07:30 10/22/23 (1)
  and the measurements would normally be considered the same, since it was manufactured as a 1193 but named E1148 by Hytron. But is measures a lot lower, though Performs the same and it seems it measures as Hytron intended but I can't find any documentation from Hytron on why they lowered the GM.
  
  iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.
  - RE: Yes, the E1148 is a direct replacement for the 1193/CV6 tube. . . - Triode_Kingdom 05:56:24 10/23/23 (0)
    "it was manufactured as a 1193 but named E1148 by Hytron"
    
    How do you know this? I don't have the Hytron data sheet, but I found one from the "Ministry of Supply (S.R.D.E.)" for the CV6. You originally referred to this tube as a "1193/CV6/6S5S," and said the heaters all draw 300mA. However, the CV6 data sheet states 200mA and Gm of 2250 to 3750. Honestly, there are too many contradictions in all this for me to make sense of it.
RE: How does a lower current for the heater at the same voltage affect GM? - Chip647 18:31:54 10/22/23 (1)
Lower heater current boils fewer electrons off the cathode if your cathode has the same formulation/size/heater structure.

There is the life test on some heaters which simply cuts the heater voltage a certain percentage.

Lower heater voltage can extend the life of the heater. This could be the purpose in a military spec tube, to make them last longer in a war.

Am I missing your question?
- I understand that but . . - John PA 19:19:55 10/22/23 (0)
  the voltage is the same. 6.3 volts but the current draw of the heater is 200mA vs 7193 specs that call for a 300mA. Yes, if designed for the 200mA it should last longer but the tube also measures lower than the specs call for. That is what I am wondering about, the measurements. Having measured almost 100 NOS now, and some previous E 1148 NOS, they fall in the range of 2000 to 2300 GM where as a typical 7193 is 2700 to 3100.
  
  Since the cathode is shorter on the E 1148, I think the lower mA is for the shorter heater, as less current is needed for the shorter cathode but at the same time, the way Hytron made the tube, as a 7193 type, the measurement is also lower but there are no notes etc., from Hytron regarding the lower GM measurements.
  
  iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.

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