Anti-matter discovery before 1932?

[Aidyan]

TL;DR Summary

A question of historical nature about the discovery of anti-matter

I once heard that anti-matter has been observed long before 1932 (the year of its official discovery) in cloud chamber tracks but was not recognized as such. If I remember well it frequently appeared in the photographs of cloud chambers with its characteristic pair production phenomenon but it was dismissed because nobody knew its meaning. However, despite searching for this I could not find any historical document confirming this. So, I'm wondering if this is a true story or is it me who has misunderstood this? If so, it would be nice to find out for the photographs of this before 1928. Can anyone help?

 

[nrqed]

Summary:: A question of historical nature about the discovery of anti-matter

I once heard that anti-matter has been observed long before 1932 (the year of its official discovery) in cloud chamber tracks but was not recognized as such. If I remember well it frequently appeared in the photographs of cloud chambers with its characteristic pair production phenomenon but it was dismissed because nobody knew its meaning. However, despite searching for this I could not find any historical document confirming this. So, I'm wondering if this is a true story or is it me who has misunderstood this? If so, it would be nice to find out for the photographs of this before 1928. Can anyone help?

In the book Second Creation by Crease and Mann, they mention that two British experimenters were upset whe they realized that they had observed the position before Anderson and had even been told by Dirac what it could be but they had waited too long to make sure of their result. Although I don't know exactly when this happened. I would not be surprised if many people, once they knew about the discovery, went back and realized that tracks they had seen in photographs were left by positron.
The book gives the following references when mentioning the two experimenters:

Blackett and Occhialini, PRSA 139, no 839 (3 March 1933), p.699

and

Dirac, Development, op. cit. pp 59-60 (same journal as above, I guess Dirac mentions there that he had talked to these two experimenters)