Theresearch that I have done is partly (a big part) as a result of the tremendouswork that the Mormons, Church of Latter Day Saints have done.The have gone into the little villages, as well as big towns and microfilmed the church and civil records when possible of these places.Lucky for us they have donw a great job in Mexico.The films are ususally done in some sort of order.For instance it could be that the Church (Catholic) had a separate book to record baptisms, marriages, deaths, confirmations, holy communion.Sometimes the records are mixed and a film may have several of the above in one book.The civil records system (government) of tracking the marriages, deaths and births, did not come into play until much later (1860-1870).Since it was a new system it took a while in some cases before it was instituted.Usually by 1870 most towns and villages were using this system....as far as I know it was a requirement.Fines were issued for not complying within a certain period that you were allowed to register births, deaths and marriages.The Church continued to records the usual sacraments...The civil records often serve to confirm the Church's records.If your ancestor was not Catholic, then the only source you may have is the civil records.In theory a copy of the civil records would be kept at each states capitol.
SLP is an abbreviation for San Luis Potosi, where I believe the Aleman's of Cruillas came from before moving there.
Family History Centers are run by the Mormons in many cities.What you do is refer to a catalog to see if there is microfilm available for a certain town and time period.You then order the film (minimal cost of 3.25 or so per film, you pay like 15 cents per sheet if the record is on microfiche)and when it comes in you are allowed to view the film/fiche on their equipment called "readers" but only on their premises.Usually 30 days is the rental period.You can renew the film for the same amount if needed for another 30 or 45 days.This gives you time to study the records.They have special copiers for the film if you need to make a copy of a record to document your research.You can't copy everything....way too expensive, but you can copy certain key documents that you would otherwise not be able to see even if you went to the location where they exist.Many of the original documents that were microfilmed are in very bad shape due to age, the elements and other such inevitable things.You are no longer allowed to view these books in person as the pages are crumbling.So you see how fortunate we are that some of them have been filmed.
If you know your grandmother was already in TX by 1920, then it is unlikely that you would find her after that time in Cruillas.
Do you know how old she was when your dad was born?This would give you some idea of the records that can be searched for her baptismal, or civil birth record.I'm guessing she would have been born about 1900 or late 1800's depending on whether your father was the first child or the last.
They may have left because of the revolution.Those were scary times for all, but many stuck it out and stayed.