There are decisions that must be made for a few items about this guitar. One of the items is securing the original bridge to remain in place during operation of the vibrato, in a non destructive manner:
(1) The bridge had separated from the plastic glue that secured it to the pickguard for vibratio operation.
(2) The plastic glue forms a melting bond with the plastic creating a mechanical glue bond and a chemical glue bond as well as shearing strength by it's chemical bond to the plastic pickguard. Shown on Work Page 4
(3) The bridge could not be reglued over the hard and distorted (3 dimensionally rough) old plastic glue.
(4) The original plastic glue's old dried chemical bond with the pickguard could be construed as damaging the pickguard by distorting it's surface.
(5) It took me about 12 hours work to remove the old glue by repeatedly masking, sanding, compounding and refacing the pickguard and bridge bottom.
(6) If the same type of chemically bonding plastic glue is used again, the same problem will exist if / when the bridge separates from the pickguard again.
(7) Intonation position of the bridge would change with changing to different string gauges.
There are 2 best options for replacing the / a bridge in a manner that will stabilize the bridge onto the pickguard for operation of the vibrato:
(1) Use a modern high quality glue that will form a strong mechanical bond not needing a chemical bond, to glue the original bridge into place once intonation position is determined and marked with a masking tape perimeter. Franklin Tite Bond is the glue preferred by many if not most Luthiers and Guitar Technicians to bond wood to plastic and vice versa. CLICK HERE to see Franklin Tite Bond properties published by the #1 Luthier supply company, Stewart McDonalds, and HERE for it recommended by Dan Erlwine for dissimilar materials bonding. I would put a strong strip of tape onto the bridges position, custom cut the same size as the bottom of the bridge, and glue the bridge to the tape. That way the bridge would stay in place but could still be peeled off, yet the bottom of the bridge would still need cleaning the tape and glue off to reset the bridge in the future, such as changing string gauges, or even changing to a different string manufacturer to a lessor degree. Glue is just not the best way to solve the problem of having a bridge that will operate with the vibrato and still be easily moveable or intoneable for different gauge and even different manufacturers metal formulas for same-gauge strings.
I DO NOT WANT TO GLUE THE BRIDGE DOWN IF A RETROFITTED ROLLER BRIDGE IS GOING TO BE MADE / USED. If I am to glue the bridge on, then I will do it with the tape method said above, unless directed otherwise.
(2) ROLLER BRIDGES: The second option, For playing (vs collecting), is to replace the bridge with a roller bridge that would not need to be glued down. On this page you can see several ways that this could be done quite reasonably:
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I happen to have a NOS 1960's / 1970's vintage Gretsch floating bridge with tuneable, individual roller saddles. A Luthier or good Guitar Tech can modify this bridge to work well on this Harmony Silvertone 1488. The Gretsch bridge has all the critical parameters required to modify a retrofit to the ST1488; But it is expensive ($100 as-is or modified as shown below). There are other less expensive alternatives said in the page's information. I will use the Gretsch roller bridge in the examples of alternatives shown below: