Small "Strange Event"

Hello... I'm new to this forum... I'm a World collector for more then 15 years... I recently bought a small collection to a friend (from his late grandfather that died in the 80's)... there was a small group of stamps from South Australia and Queensland...

I was classifying them and I stumbled in this SA QVictoria stamp, with the left frame missing...I think it is the SG 175 (perforation 13 and Crown above SA wmk).. I think that the wmk have the S and A close together, so, this was the only option... SG says that it should be green or pale green but it is in fact a blue green stamp..the wmk was clearly visible , so I will upload an image of it...

My questions for you are...
--is there any knowledge of this event..?
-- how did it happened.?. what was the process.. what could have happened during the printing?

Thank you..
Regards

Shades vary in Australian States a lot. I also have pale green, green, and bluish green stamps of this S. Astralia issue.

The left frame covered by ink is an interesting printing error but if Gibbons doesn't mention it as a constant plate flaw then it is, probably, a one-time "over-inking."

I don't have any more detailed reference for Australian States than Stanley Gibbons. Perhaps, experienced Australian collectors could tell you more.

I found this one. Will have to do some research on it.

Asked an expert in Australian States and he said this is not uncommon for this era.