The devices made by IR are 100% not symmetrical MOSFETs. The gate and source are on the top of the device, while the drain is the bottom of the wafer, which, while I can't discuss fabrication details, has an ohmic contact with the package.
So, gate/source is very low cap while the drain is many microns away, so should have much higher capacitance.
No effort at all for a groove-type symmetry. This is strictly a planar device.
Rleff (nice, diffusion oriented name).....I'm not knowledgable about probe enough to know who makes what. The last HP stuff I saw in a test area were the frequency meters used to tune quartz crystal oscillators. This was no later than about '80.