The serial number says June, 2009 production. Just at the end of its three year warranty time.
The guys are correct, the + upgrade changed from 6N1P tubes to 6CG7 tubes, requiring shorter board spacers and low profile tube sockets to make this possible with the cover on. Prior to that some mother board wiring was changed to allow the use of 6V tubes rather than the original 12V tubes.
By the way, I have heard from a very reliable source that the best tubes for the T8+ are old new stock clear top RCA 6CG7 tubes.
Regards,
Frank Van Alstine