All slot to slot measurements are outside edge to outside edge see here
Please measure webbing width at the slot as it will have stretched

2 different sizes of daybed kits models 355-1 and 355-2,
models 357 and 358 single beds kits fit both models
model no. 355-1 were made from October 1956 to November 1972 and again from July 2010
model no. 355-2 were made between December 1972 and July 2010