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That's normal for this vintage of board.
Back when these boards were made -- the boards were plated before solder mask was applied. This plating melts while the board is being wave soldered resulting in what you see.
Now, when boards are made - the boards are etched, solder mask is applied and then the board is plated -- no plating beneath the solder mask (process is called 'solder mask over bare copper' or SMOBC). New boards do not exhibit this 'bubbling' like these old boards did.
Ed
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