As pointed out by Hiraoka, several late Edo texts feature illustrations of that gravestone and its inscription. Let's look at three different works mentioned in the articles first.
1) Shiba Kōkan's 司馬江漢 Saiyū ryodan 西遊旅譚 (1803) @ https://t.co/32ORuuiMB5 3/ https://t.co/OmLMTq3CtK