just measured mine which is an all-red-socket version, but not a very early one. I was surprised to see some variance, but they were all 7.5-7.8mm
Mine is an early all red socket. And I can confirm today very similar numbers. So they seem nominal, i.e. were not pushed in at any time...
my 34401a terminals depth
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early model, all red terminals, which has the failing VFD driver ('unobtainium')
front terminals
recess distance from face of plastic lip to lowest edge of copper ring
1. HI+ = 7.51mm = 0.296 in
2. HI- = 7.34mm = 0.289 in
3. LO+ = 7.62mm = 0.300 in
4. LO- = 7.46mm = 0.294 in
5. I3A = 7.30mm = 0.287 in
plastic hole max inner diameter = 8.20mm, typical = 8.20mm = (imperial) 0.323" max, 0.323" typical
copper ring terminal max inner diameter = 4.18mm, typical = 4.10mm = (imperial) 0.165" max, 0.161" typical
rear terminals
recess distance from face of plastic lip to lowest edge of copper ring
1. HI+ = 7.56mm = 0.298 in
2. HI- = 7.42mm = 0.292 in
3. LO+ = 7.97mm = 0.314 in
4. LO- = 7.79mm = 0.307 in
5. I3A = 7.36mm = 0.290 in
plastic hole max inner diameter = 8.30mm, typical = 8.20mm = (imperial) 0.327" max, 0.323" typical
copper ring terminal max inner diameter = 4.00mm, typical = 4.00mm = (imperial) 0.157" max, 0.157" typical
Not sure that helps anything, seemed like I needed to measure mine now and check how bad they were. To select the right type of jacks (for getting the probe master leads too).
To check against the probe master compatibility list, and the dimensions / specs on their CAD drawings.