From the data I have a 53-56 would be a 40c, a 56-58 a 420c and from 58-60 a 430c,, the 40c used the 100 cid engine, the 420/430c used a 113 cid engine,, both of these were 2 cylinder gas engines. Also available from 58-60 was the 440 with the 113 cid 2 cyl gas engine and the GM 2-53 diesel, the 2-53 is definetly a sleeved engine. Is it possible the current block was worn beyond +.125 (biggest overbore kit I could find) and sleeves were made to repair a worn out block? Im curious because I dont find any info on a factory sleeved 2 cylinder gas block, not saying the data I have isn't incorrect or all encompassing by any means. Pics of the machine, engine and the sleeves once removed would be nice.