The PDB documentation for plug-in-displace shows:

    { GIMP_PDB_INT32,    "displace-type",  "Edge behavior { WRAP (0), SMEAR (1), BLACK (2) }" }


The lines up with the radio group:
  frame = gimp_int_radio_group_new (TRUE, _("Edge Behavior"),
                                    G_CALLBACK (gimp_radio_button_update),
                                    &dvals.displace_type, dvals.displace_type,

                                    _("_Wrap"),  GIMP_PIXEL_FETCHER_EDGE_WRAP,
                                    &wrap,
                                    _("_Smear"), GIMP_PIXEL_FETCHER_EDGE_SMEAR,
                                    &smear,
                                    _("_Black"), GIMP_PIXEL_FETCHER_EDGE_BLACK,
                                    &black,

                                    NULL);


but when this plugin is called programatically the value is passed directly to the pixel fetcher:
(line 559)  gimp_pixel_fetcher_set_edge_mode (pft, dvals.displace_type);

and this is an enum type (from gimppixelfetcher.h):

typedef enum
{
  GIMP_PIXEL_FETCHER_EDGE_NONE,
  GIMP_PIXEL_FETCHER_EDGE_WRAP,
  GIMP_PIXEL_FETCHER_EDGE_SMEAR,
  GIMP_PIXEL_FETCHER_EDGE_BLACK,
  GIMP_PIXEL_FETCHER_EDGE_BACKGROUND
} GimpPixelFetcherEdgeMode;

So the PDB call doc needs to be changed to:
    { GIMP_PDB_INT32,    "displace-type",  "Edge behavior { WRAP (1), SMEAR (2), BLACK (3) }" }

to line up with the enum type.