Greetings! Just joined because I found a thread concerning these incubators with an online search. (Duck Duck Go - NOT Google!). Were/are these incubators insulated with asbestos? Thanks in advance…
If you never get an answer from anyone with one of those, and you absolutely need to know, you could take an extremely small drill bit and drill through each suspected side, being sure to avoid the one with the wiring going through it. The drill bit would pull out a small sample of whatever is in there, and if it didn't, you could insert a small wire with a tiny hook on the end.
You could then plug that little hole or leave it for ventilation.