The thickness is the same and the best way to verify this is to check out the C-IV images for the ESB (the front corner of the perspex block is hanging out a bit) and the Heilman images for the ROTJ (the perspex block is also hanging out from the RF casing), the only major difference is the front cut angle (around 50 degree for ESB and 64 degree for the ROTJ), the lamp channel diameter and length (?) and the triangle is a bit bigger on the ROTJ (with 0.5mm), but again these are not set in stone and further research might change these values.
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I can't verify that the perspex are the same or different in thickness, but will say that they are clearly hand cut so there are differences.
Also, I think it is highly unlikely that the drill holes or drain of wheat bulbs would be different from helmet to helmet. There might be slight variances in the perxpex, but I would tend to believe those aspects are the same.