The .52A is the current version, some folks use the "A" in their product descriptions and some don't.

The Magnum XLS .52A is a dual ball bearing 2-stroke that is "ABC" rather than ringed construction. Most modern ringed engines are also ball bearing engines. Most modern plain bearing engines are "ABC" type construction rather than ringed.

The Magnum XL .52 RFS is a ringed ball bearing four stroke engine. Two engines, same displacement, same manufacturer, very different designs.

The Magnum line of engines are manufactured in China by Sanye. They are imported by Global Hobby, and even Global's own marketing refers to their .52-size 2-stroke as both the .52 and the .52A:

http://www.hobbypeople.net/gallery/210760.asp


Magnum – Bringing you high performance at an affordable price for more than 20 years! During that time, Magnum has grown and expanded to offer a full line of 2-stroke and 4-stroke engines, made from the finest materials on the latest machinery. Plus, you get the added security of a full three-year unconditional warranty that protects you from any manufacturing defects and/or premature wear (crash damage not covered). For reliability, high quality, and performance the choice is MAGNUM!

The XLS .52A is a single-cylinder two-cycle engine incorporating ABC (Aluminum piston, Chrome plated Brass sleeve) technology for long life and easy break-in. A dual needle valve carburetor for precise fuel/airflow metering is standard. The XLS .52A features an optional cast aluminum rear needle valve assembly and high-flow muffler for more power and reduced noise. The Magnum XLS series engine was designed by expert engineers and built by master craftsmen using only the highest quality materials and CNC machinery. These qualities provide the long life and dependability you have come to expect from an engine of this caliber.