This is simply whisky and water, but as with the Japanese tea ceremony, observing the time and care taken over it making and the prolonged anticipation...
I apologize for taking so long to clarify, I only saw the reply today. Under variants, there's one named "Shunbun" which mentions ume shiso. Thank you!
I am guessing based on Simon's results specifying a liquid, its probably referring not to a garnish of ume and shiso[also called perilla], but to a product like Umeshu[plum wine] brand Choya Umeshu Shiso. They also produce a Choya Umeshu Honey, which appears to be the product the "Risshun" refers to.
Sadly I don't know. Hopefully, another reader can help. A search reveals many references to a Hibiki Shunbun "25ml Ume shiso (Japanese mint)". This is a very old page and I don't have related notes as to the original source.