This is really a subject for an expert on the details of Catholic religious practices in Spain and Spanish America. As I recall (and I studied this a long time ago) the Council of Trent, which convened three times between 1545-1563 standardized the Mass and many other religious practices throughout the world. I believe they also ruled that it was not necessary to receive both "bread and wine" and that the Body of Christ was fully present in the "bread" which was much easier and safer to use.
I have no information on the exact form and nature of the communion wafers.
The submitter did some further research with the diocese and wrote this follow-up:
Thank you again for your response! I do appreciate it. I did a bit of digging today, and discovered a few interesting facts that might relate to the communion question. I believe it is standard practice that one can’t take communion unless one is confirmed. Confirmation could only be done by the bishop, or by the Father Superior of the Franciscans in California. So probably not everyone could take communion. The bishop appeared but rarely (never even visited Purisima mission until after secularization), but Father Lasuen confirmed about 1000 people during the time he was approved to do so, and Duran also confirmed during his tenure. I noticed that at times, baptism and confirmation occurred on the same day (!), probably because there wouldn’t be another chance (had to be done while the approved authority was around).