Welcome to tfl!
I'd like to help, but can't do much. The key is finding out who made the rifle for Wards, and what model it is.
I did a couple of quick searches, and found a couple different tables listing the trade guns and who made them, but I now have doubts about the accuracy of all entries...
one said the Hawthorne 558 was a Mossberg 500. The other didn't list a Hawthorne 558 but did list some other Hawthorne models.
It did list a Western Field 558, but said it was (also) a Mossberg 500.
The only Mossberg 500 I know is a pump shotgun (and a very popular one).
so, stone wall there, sorry.
Your gun might be a Mossberg, or a Stevens, or a Marlin, or something else entirely. Once it's ID'd we can search for the part, but that doesn't mean its available. Some parts are only available by finding and buying the same make & model gun, tog get the part.
It's possible the safety from a closely related model might be fitted to work.
It's also possible one will have to be made. A good smith can do this, but its possible the cost of his labor (time) could exceed the value of the gun.
I did see an ad during my search, for a "Hawthorne 558 rifle", but it had been sold and there was no picture. Sold for $35.
Good Luck! If I come across any useful information, I'll post it.