If you build a cabinet for 817 with 515b woofers, can the woofers be swapped to 515G or vice versa?
Of course they can.

Later 515B is built on the same 16 inch frame as 515G.

The 16 inch frames have slots rather than holes for the mounting bolts such that they will work with a range of different bolt circles.

The slots and the bit of forgiveness they offer in getting the bolts started in their respective T-nuts is well appreciated. Those fellas who have loaded 515's into 210 cabinets without help know exactly what i'm talking about.

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they list the bolt hole diameter at 15 inches. and in the 816 at 15 1\8. i never knew this till i looked it up this evening. i have 816 cabinets at work and will verify this tomorrow. which is funny since they give a listed output of a 515b in a 816 cabinet
After some point in 1975 515B looks like the one in this pic:

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