Here is an update to this issue and the troubleshooting steps I have taken:
1. Delete and re-add troubled Insteon devices -- results unaffected.
2. Revert back to 5.0.11 of UDI ISY994i firmware -- all of the devices contained in my v4.x.x were working as before but adding any previously non-working Insteon 2477 dimmers into the ISY resulted in the same error as described above. Oddly, some of the newer additions work perfectly but others do not. The use of the switch was reported correctly (as far as I can tell) into Indigo but Indigo could not control the device and the "ISYtype" undefined error occurred.
3. Revert back to latest official UDI 4.x.x firmware -- same results as #2
4. Return to 5.0.13A -- the same results as #2
5. Update to 5.0.13D -- the same results as #2
6. Delete the non-working dimmer from the ISY and add it to Indigo directly -- works perfectly as expected.
In each instance I tried starting on the ISY from factory reset and adding devices and from backups performed either historically or just prior to starting this troubleshooting exercise but the results were the same.
I purchased two lots of dimmer switches from Smarthome about 3 weeks apart this summer but cannot tell that anything is different between the boxes or how they are reported within the ISY Administrative Console. In other words, if Smarthome has updated the internal firmware of the switch, I cannot tell it from the switch, the ISY or Indigo.
I've written a few Python scripts but I am woefully deficient when it comes to coding. I get the logic but not necessarily the language. That being said, I can't seem to see how the dimmers are being defined differently between the working devices and the non-working ones. The error when I attempt to control a dimmer with the ISY Bridge Plugin is always the line 520 "KeyError: key ISYtype not found in dict" in the log.
I'm beginning to think that I might have some dimmers that have newer firmware and are reporting themselves to the ISY differently but I don't know how to discover if that is the case.
One might wonder why I don't just eliminate the ISY and stick with Indigo or vice versa. I find that my ability to write simple scripts and report data (like my heating oil tank volume) much easier and more intuitive in Indigo. However, the ISY is much quicker for me to to add Insteon devices and to make scenes than Indigo and I use IR on the ISY to control lighting with my home theater universal remote. I have both and want to utilize both for their advantages if I can.
I would be happy to post more error logs or debug logs if anyone looking at this has some ideas. Thanks for reading.