Pocketbook still has repos containing old versions of their SDKs out there if you search for them. @brucelee put together a nice package of old and new SDKs in his repo at
https://github.com/blchinezu/pocketbook-sdk. That is the one I would recommend using. You should also check which version of the PB360 you have before building for it. The original ones were version 1 firmware devices and you would use the PBSDK version when building for it. The last models were version 2 firmware devices (they were actually part of the Pocketbook Pro family of devices). For those you would use the FRSCSDK version to build your applications. SDK_481 is for the version 5 devices that came out long after the PB360.
The PB360 devices were iconic, and I'm glad to hear that some people are still getting some joy out of theirs.