The Oxnard-made Guilds seem to have serial numbers that are date coded where the first two digits denote the year. So, in this case "18" denotes 2018.
No, not in one year, it is a running total since they started production in Oxnard, similar to what Martin is doing but adding the year to it.C means Oxnard, California, yy is the last two digits of the year and xxxx is a sequence number used to differentiate instruments made in the same year?