They are Associated shocks with Yokomo Bladder Caps (a similar system to what Tamiya uses in its TRF shocks). You can get the caps through Speedtech R/C or any Yokomo dealer.
I think the bladders help you build more consistent shocks, and I find I've gotten better results with them. There's a fellow I race with who has studied fluid dynamics and can go on about how you can use bladders as a tuning option, but I think that's far beyond anything we need.
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