Last week I bought another piston but I never seen this.
So why the difference 
John 
email (option): john.kater@chello.nl
Hi John,
As Ron says its a split skirt piston. Before you bore the cylinder you neeed to know the piston makers recomended clearances. You can run some of these pistons with a very small clearance even down to 0.002" depending on the type and make. If the piston top is not marked Front make sure the split goes to the front of the bike, this is so it is away from the downward thrust which is on the back of the cylinder.
I do not reconize the piston type in your photos, what is it to fit, it's not BSA.
Tim W
email (option): t.j.walker@btinternet.com
Well Tim. this is a FRIGOBOR piston for my BSA. The marking on top is 83,52. I bought this because the weight is 414 grams and my current Heplex is 451 grams.
Thanks for the info.
John
email (option): john.kater@chello.nl
BSA split skirt M33 pistons (85mm bore) run a bore clearance of .0006"-.00275"...
Hepolite etc. made aftermarket split skirt pistons for M20/21 engines and I would think if you went with .002"-.0025" clearance you should be OK...
Piston ring gaps remain unaltered...
Of course, take care when first running the engine to ensure everything is OK before commencing normal running in procedures...Ian 
email (option): ian@wright52.plus.com
As Ron says new unmarked pistons can be fitted either way...
However, it's worth noting that even an unmarked piston, if it has been removed from an engine after use and the intention is to refit it, should be refitted the same way round...Ian
email (option): ian@wright52.plus.com