I don't think its posted so that it has any legal effect , IMO
There is no legal requirement for sign placement, size, or wording in OK. It is posted, that is all that counts.
The legal effect that it has would be that of trespassing, not a violation of any firearms law.
I would think that sign placement has a requirement ... it must be able to be seen by a reasonable person...they cannot expect people to get on their hands and knees to read one...