16A07 - year, cross, 7th seedling to be kept.
I like to put the year first, because that way I find it easier to tell if I am looking (in my notes) at new seedlings, seedlings which might bloom or seedlings that have been saved through 1 or more rounds. I use a letter to indicate the cross, as I will not(!) make more than 28 crosses (numbers of letters in the Danish alphabet) in a year. I only assign a letter to succesful crosses, but keep a record of unsuccesful ones too - just to educate myself on what does not work or is difficult. I too only give a number to seedlings which are not composted in the first round.
PS: My seedlings from 2014 has a 'Je' in front of the seedling number and no letter to indicate the cross (e.g. Je1403) as those seedlings are grown from seeds I bought ('Je' indicating the seller) and I have no information on what crosses were involved.