Thanks everyone for the info and suggestions - great community here. Not 100% this was an Adtran only feature, but I do believe this was something vendors were doing years ago when fiber links were young; perhaps as a cost saving option. Best I can see the industry has settled on the existing tiers of 1G, 10G, 40G, and 100G and I can't find any current vendors selling this. So, for me, it's a question to the client regarding upgrading their physical fiber to support 10G and moving forward. Since I have the 2.5 SFPs I might try them in my new switch, just to see what happens, but I'm not expecting them to function.
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