I took some measurements of a stock Gen 4 and Gen 5 backplate along with those of a Gen 1-4 SCD (Tau manufacture) and Gen 5 SCD (LTT manufacture) with a digital caliper. Hoping this will help some who are wondering if they can use a Gen 5 SCD on a Gen 1-4 Glock. Sorry for the crappy cell phone pics and if my handwriting isn't legible. Let me know if needed and I can type out the measurements or try to get some better pictures.
Attachment 86248
Outside of the obvious difference in the shape of the right side notch, what I'm seeing is that the notch on the left side of both the OEM Gen 4 backplate and on the Tau Dev Gen 1-4 SCD is slightly larger than their Gen 5 counterparts (.01" larger comparing the OEM backplates and .0135" larger comparing the SCDs). Right side notch on the Gen 5 SCD provides plenty of clearance for Gen 1-4 Glock frames.
I installed the Gen 5 SCD on a Gen 4 G19 to test fit and function/compatibility and it does work but is a little tight due to the narrower left side notch on the Gen 5 SCD. It runs into a slight clearance issue on the inside edge of the Gen 4 ejector housing and what appears as a bit of a flange on the rail (this same flange is not present on a Gen 5 ejector housing). A slight push on the gadget will push it past the flange but it will not swing back to the fully closed position freely.
Outside of the stickiness when pushing the SCD forward, function is unaffected in the sample of one that I've tried it on. Speaking only for me, if I had a Gen 1-4 gun and the lack of an SCD was preventing me from carrying it then I'd grab the LTT gadget and slightly file/sand enough material off of the ejector housing flange to make it flush with the remainder of the rail and use the gadget without concern.
One final thing that I've noticed on both of the LTT SCDs that I picked up, the flange that fits into the slot in the backplate is around .005" thicker and the hinged plate is around .003" wider. It isn't much but I've been able to tell that they're a more snug fit than my Gen 1-4 Tau Dev SCDs and exhibit less play during install/removal. Overall awesome products and I'm ecstatic that they're back on the market as I was in the boat of not switching to carrying my Gen 5 until I got a gadget back on it.
Flange on the Gen 4 ejector housing rail that causes slight interference with the Gen 5 SCD
Attachment 86250
Gen 5 SCD on Gen 4 19
Attachment 86251