This root canal was originally done in 1965. A periapical lesion has developed. While the root canal filling is weak and the apical seal obviously an issue, the tooth is and has been fully functional.
It is likely that the tooth had an apicoectomy, due to the short length of the root and open apex.
The open apex is debrided and a new apical stop is created.
The canal is then obturated with MTA. MTA is chosen as the obturation material due to the open apex and the ease of future apical surgery if needed. I call this a "root canal upgrade". Preserving this tooth preserves the periapical tissues and helps to maintain the bone around the tooth. While there are lots of good replacements for missing teeth, nothing preserve the periapical architecture as well as a healthy tooth & periodontal ligament.