The LM-122 makes it an L-series (L) Metro (M) van, in 3/4-ton (120, 121, and 122, depending on actual GVW of the van). While the L-series trucks were generally designated as 1950-'52 model years, some were titled in the year they were sold, rather than the year they were built/designated by the factory, in some states. So yours may have been built as a '52, but not titled until '55. So try '50-'52 year models for LM parts and service manuals at Binderbooks (service manual will be the CTS-11, but there may also be a Metro body supplement available).
And the SD-220 is the engine designation. The SD engines were available as 220 and 240 cubic inches, and they were International-built sixes with Delco-Remy electricals (starter, generator, distributor). So the SD-220 is simply the engine designation, and the make is International.