When did readers learn that Wolverine has such feature? Was it known from the very first appearance of Wolverine in 'The Incredible Hulk #180' from October 1974?
TL;DR. We find out that his skeleton is made from adamantium in The Uncanny X-Men #126 from 1979.
Wolverine's first appearance was a single panel cameo at the end of The Incredible Hulk #180.
There's no mention of adamantium until the following edition, The Incredible Hulk #181, where we learn that his claws are made from the substance.
Interestingly, in the same issue he takes a blow to the head which he's described as surviving only due to his "astonishing speed and stamina". No mention is made of an adamantium skeleton
It wasn't until X-Men #98 that they're revealed to be part of his body (as opposed to part of his suit).
In X-Men #124 we learn that Wolverine has "unbreakable bones"
and in The Uncanny X-Men #126 we finally discover that his bones are made out of a metal called adamantium