Master Cylinder Pushrod Retainer Question

I have a 67 Barracuda with manual drums all around and a leaky non-original master cylinder. The replacement I chose is a reproduction of the 60’s Bendix drum/drum unit from Mancini Racing MBM Bendix Drum Brake Master Cylinder. The product description doesn't specify power or manual brakes, so I assumed it works for both. I got it yesterday and it looks identical to the manual MC pictured in the service manual.
Here is the problem: the bore in the back of the piston doesn’t have a groove machined in it to catch the rubber retainer on the push rod.
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The original looks like this:
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And the new one looks like this:
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I was under the impression that all manual MCs had this groove in the bore to hold the push rod in. Maybe the unit I got is for power brakes? Is it safe to use without something physically holding the rod in the bore?
Also, the piston retainer on the new unit is just a little tab in one of the screw holes, where the one that came off my car uses both holes, has a rubber boot, and looks like this:
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Can I use this on the new master cylinder?