The U-bolt Vanguard, developed by IvanTheTroll and released in mid-2020, stands as one of the early reliable models of reinforced AR-15 lowers. This design was the first to incorporate the reinforcing U-bolt, which now features in several other AR lowers like the Firebolt. This reinforcement allowed the lower to be printed in PLA+, which at time of release was quite uncommon.
The lower itself is a single-piece design, based on the earlier Vanguard series of lowers. At time of release it was considered to be the strongest printable AR-15 lower design, withstanding several rifle pushups. It still stands as a solid, reliable and easy to print AR lower design.