
Usualy first 6 digits are TAC and the following 2 are FAC, which are than followed by 6 digit serial number and last digit is Luhn Check algorithm, which determines the final digit in an IMEI number, I am not sure for USA AT&T one, but if they use IMEI than it should be the same
European Pre 3 has TAC 355825 and FAC 04.
As it seems from your post HP Pre 3 from AT&T have
TAC 012824, FAC 00 followed by serial 6 digit 0XX4XX and Luhn Check algorithm X
TAC 012824, FAC 00 serial 0X7XXX Lun X
 If looking this number TAC(012824) FAC(00) SN(0XX4XX) LUHN(X) than 0XX4XX would mean XX4XX pcs if the two digits preceding nr. 4 the ones you marked as XX would be different than 0, than the maximum would be 99.999 pcs, if the two digits preceding nr. 4 the ones you marked as XX would be 0, than the number of produced units would be maxium 999 pcs
 If looking this number TAC(012824) FAC(00) SN(0X7XXX) LUHN(X) than 0X7XXX would mean X7XXX pcs
if the digit preceding nr. 7 the one you marked as X would be different than 0, than the maximum would be 9.999 pcs, if the digit preceding nr. 7 the one you marked as X would be 0, than the number of produced units would be maxium 9.999 pcs
If you let me know the correct first digits I am talking about than I would know for sure (0XX4XX, 0X7XXX)


