Simply, this.

Reynolds 520 is identical in specification to Reynolds 525 except 525 is made by Reynolds while 520 is made in Taiwan to Reynolds specifications. Note in the tables below that the chemical composition of Reynolds 525 is the same as AISI 4130. However, that doesn't mean that a frame tube meeting 4130 spec is the same as a Reynolds 525 (or 520) frame tube because tubes can have different wall thicknesses, butting profiles, some can have welded seams while others are seamless, etc., so ride characteristics can vary depending on these differences.