I need help identifying used MTB tire sealant
The final result was "don't do that" (where "that" was to try to identify and match the existing sealant). Per advice in the comments I cleaned out the remaining liquid sealant (and the crud where the sealant had pooled and hardened during non-use), cleaned the inside with isopropyl alcohol and a lint-free towel, and then set the tires up with the sealant I have on hand (Stans). They have been holding pressure and riding great.