try part no. MS20470A-6-6 this a pure aluminium rivet with a universal head [very unlikly to find MS20435 round head which land rover use] well i got that wrong.
3/16 inch diameter[ie 6/32nds] 3/8 in long[ie 6/16ths] the A is for the material 1100 soft.
i have just ordered a 1/4 lb of -6-9, which should appear on my doorstep tomorrow.
so if anyone up this way needs some call me.

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Safe Travels
harry