Good guess that it would be referred to as Rockwell in that era..........but the history is Timken to Rockwell-Standard to Meritor.  A picture of the flange end with outside diameter and bolt circle with # of cap screws would help . No junk yards in your area ?   It was common for a yard to have a stack of axle shafts .

The axle shaft will not necessarily all be the same in one axle model. So forget about figuring out the exact model. Start by looking on the flange of the broken axle for shaft numbers.

 Rockwell axles often have (  ) around the center of the flange, while most other mfg are flat  faced. 
 You'll need to know spline count, length and flange diameter and bolt count/circle. 
 Many axles had a course spline in early years and have gone with a fine spline in the same axle model.
 For example, you can have a IHC 472 with 20 spline or 39 spline, you need to know what you are looking for.
If Rockwell axle you can find some of the half shafts in the attached manual.
 Foote axle is another source once you know what you need.  

Is the digger a backhoe or telescoping boom excavator? Hopto originated in Winona, MN in the late 1940s as a backhoe attachment for farm tractors.  With their success they developed a telescoping boom excavator.  So did Gradall, and Warner Swasey bought both companies.  Trying to merge two competitors did not work well.  I don't know what Duplex and Hopto might have been, but could be a Hopto hoe on a Duplex chassis?

I worked at Badger Construction Equipment (corporate name for Hopto) in Winona 1991-2 as a service rep.  At that time (and much earlier) Badger built their own carrier chassis from scratch and I think used Rockwell axles.  Service work in 1991 was focused on hydraulics and transmission misapplication troubles.  Part of my job was finding former technical training info so my job was digging into their history.  My supervisor was a service manager who worked there since the 1950s who told me about the booming years in the 1960s and the contention with Gradall.

Bruce

A picture of the unit might help a little bit. Is it a 4x4 or 6x6 chassis ? Probably the best thing you can do is crawl underneath with a scraper and try to find some numbers on the rear(s).

Jeff

Not much help here either but Sanford fire trucks also used Duplex truck chassis. I think they were made in NY.


This one with a 6V53 Detroit.



You might search the fire truck guys.


Tony

Welcome to the forum. I'm absolutely no help with this but there are many on here who can probably steer you in the right direction. They will start chiming in soon.

Churubusco, IN

Facebook: Terry Tombaugh

Hello everyone,
happen to have a hopto digger model H-434-A and have two broken axles. I am hoping to find out the manufacturer of those axles so I can hunt down replacements. I have attached a photo and ask for help from anyone who might know the manf. thanks in advance for any assistance.