Just because a lot of people are commenting on this without seeing the form, if you go here[1] you can see it in action (no association with the page, it was the first one that turned up on Google).
A couple of takeaways missed by various comments:
The hidden input box can in fact be manually edited, and if the user selects "Founding member" that fact is highlighted (the cursor is inserted into the textbox).
The hidden input's name attribute is "value". The guess that 1Password is basing its guess on the "/year" text is probably accurate.