Our arena shard has a statistics guy that is doing a huge mod drop tracking project. There are a ton of people contributing their drops to it.
Right now (and I believe we have a pretty significant amount, probably thousands of sims), it boils down to about this:
Generic Triangle (any color): ~4.5%. You could say there's a +/- and that the code may be set at either 4 or 5.
Now, if you want a speed secondary blue, purple, or gold: That's around 0.18% chance.
Shouldn't be much of a surprise but you have about a 24% chance of Circle, 24% Diamond, 22% Square. Cross is ~13%, Arrow is ~11%, and as mentioned, Triangle is ~4%.
Obviously, there's probably some deviation. If I had to guess at the code based on current trends, Circle/Diamond/Square are all 25%, Cross and Arrow are probably 10% and Triangle is 5%.
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Note: These values are as of now, it gets updated every 100 sims or so and should not be assumed to be "the gospel".