91.5 is not a standard tone for the USFS, or any other federal or state natural resource agency in California. One of the standard tones is still used to key the repeater up. The southern California National Forests (ANF, BDF, CNF, LPF) have, up to this point, used 103.5 (tone 8) as the output tone on every repeater. Many of the other 14 National Forests in the state rebroadcast the input tone of each repeater. One exception is the Sierra National Forest, where each repeater transmits a non-standard tone on the output. Locally I listen to the Shuteye Peak repeater on the Sierra. It has a Tone 5 (146.2) access tone, however, the output has a tone of 82.5. I'm not sure why this is done, but in any case there is a unique non-standard tone for each repeater's output.
The San Bernardino might be moving toward the configuration the Sierra has. I like it as once you figure out what non-standard tone is used for each repeater you can distinguish which is in use each time you receive a signal on the frequency. After 32 years in the area I have the Inyo's tones so ingrained in my head I just look at the output tone (radio set for tone search for the Inyo's frequencies) and know which repeater it is without any delay. In my travel programs for CA, NV, AZ and NM, when I know the forest or park re-transmits the input tone on the output I program a channel for each repeater, put the name of the repeater in the alpha tag as I can't rely on my memory in areas I don't know as well. .
On the Inyo, microwave linking is not used. There are two remote base radios, one for the north net and one for the south net, which are then linked to dispatch and ranger stations using UHF frequencies. The output tone of each repeater is used to distinguish the repeater in use so that the display screen used by dispatchers show them which repeater to answer with.
In this case this is not a new repeater, just a new output tone. Of course I can't say this with complete certainty from 300 miles away, but given what you observed and reported, I would be willing bet this is the case. Keep us posted, you might start hearing some other unique tones on the system if or as each repeater is reconfigured.
“We have, I fear, confused power with greatness.” Stewart L. Udall