A rubber plug in the transmission?
 

I recently started owning an H3 and I'm not sure whether this hole on the transmission (as visible in the picture) was unplugged when I bought it, or if it has recently lost it's plug. Heck, I'm not even sure what the hole is for, since there have never been any holes like that on any of my other vehicles. I now have three questions:
1) What is the name of this rubber plug and possibly a part number (so that I can order a replacement)?
2) What is the use of this hole at all? Maintenance?
3) What could be the possible harm from driving around (including in some heavy mud) without this hole plugged? What should I be worried about and what should I do to mitigate any problems?

!!
 

another first! i had to look. mine has a shiny metal cover about an inch wider than the hole,an inspection port?? its like the smaller on to the rear. the rubber strip is for access for locking the flywheel for some maintenance i think i remember reading.

id try a tranny shop b4 a dealer who will rob you but it cant cost much.
yeah,mud/water going in there is not good,
use "duck" tape right away!!

Agree. Ours both have tin disks cover caps there.

It is an inspection hole for the flywheel. There should be a metal cover for the hole.

Agree with the above mine has a metal cap/cover. It fell out once when I was changing oil I had to bend 3 little tabs to get it to stay on.

4L60E Convertidor de par de vivienda de campana Tapa De Inspección Cubierta enchufes de servicio

It's called inspection hole cap and not having it does not represent a harm for your transmission according to several forums (But it is there for a reason so you might want to replace it!)

Maybe you can do something yourself, or search the internet, scrapeyard or even the dealer.

be careful when replacing it as this part gets really hot.

Good luck!