Hi Mark, It seems you have a 50 mm heater and you need 96411‑45000 PLUG, TIGHT, 50 mm to close it.
There are a few options and sizes, but in this thread (on page 2 at this moment) sved gave the dimension and picture of a 30 mm setup so that was enough for me to order when I found the partnumber.
https://forum.ih8mud.com/diesel-tech-24-volts-systems/525082-2h-freeze-plug-block-heater.html
But please search further before you order.
-rad heater: dont want it, when the engine is warm the thermostat will open and the rad wil be heated by the engine.
-oil pan sticky type heater: dont want it, it will heat the north-pole before the engine warms up.
-inline heater, dont want it, dont know what hose to cut and I want the heater mounted in the block.
-35 or 50 mm heaters: 50mm seems to have the best and lowest position in the block, 35 aint there, 30 is.
-place of the heater:lower seems better, warm water seems to go up, ideally you get a flow.
-more things to consider?
Other plugs for sale and dimensions:
11116A PLUG, TIGHT, NO.1
96411‑42000 2H..HJ60, OD=20 3 € 3.29
11116B PLUG, TIGHT, NO.2
96411‑42500 12HT, 2H..HJ6*, OD=25 6 € 4.34
11116C PLUG, TIGHT, NO.3
96411‑43000 12HT, 2H..HJ6*, OD=30 12 € 5.01
11116D PLUG, TIGHT, NO.4
96411‑45000 12HT, 2H..HJ6*, OD=50 1 € 5.15
30 mm freeze plug, below glowplug, behind edic, before firewall, number 6 :
-need to make some kind of puller to get it out EDIT:slam with a punch and it will turn, grab with pliers and pull out, can be reused for emergency.
-have to find a way to slam it in without damage, the manual does not mention any type of sealant/stuff. EDIT: just take the brass drift used for the knuckle and gently tap it in, after every tap go to the other side.
-Maybe the 30 mm installation is a lot easier then those poor souls with an 50 mm
50 mm plug rear 96411‑45000 PLUG, TIGHT, seems impossible, or maybe you have time to remove the engine:
Maybe the radiator side is possible? it has been done, but you might also have to move the mont blanc.
This 30 or 50 mm thing is the same as knuckle repacking with special holy water absorbing-expelling grease that only exists in mad peoples minds, 50 mm seems best

EDIT: 50 mm seems impossible to install on the lower freeze-plug on cabin side. Engine has to be removed! (?)
https://forum.ih8mud.com/60-series-wagons/564438-need-block-heater-part.html