It's possible that this was made in the transitional period between USSR and Russia, and therefore wouldn't have a stamp. It's somewhat more likely, however, that the movement has been swapped out. The movement itself is still accurate for the model and is arguably one of the best ever produced in USSR/Russia. They go for about $30 now, which is much cheaper than a full service at a watchmaker; I myself have done this with an Amphibia relatively recently when it started running super slowly.
I don't think it's that simple. SU stamps appear in Russian made pieces as well, and the RU sign seems somewhat new. Still - there are some basic veriosn from late eighties with no stamps on the bridges. I think it depended on the run, the production line etc. Of course, the movement might have been replaced, you never know for sure. But I think it can be the original USSR made movement. Still - in this particular watch it makes very little difference.
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