Reverse sits on its own shaft (counter-shaft), which likely stops spinning altogether in forward gear after the lubricant warms, but tends to slowly spin when first starting the car, so, as John & other's said, without a synchronizing ring&gear, trying to go into reverse with the counter-shaft spinning, you get some grinding. If John's trick of going into 2nd with the clutch in & then keeping pedal depressed as you go to reverse works, go wiith that. Another thing that might work is to just hold the pedal in and the transmission main shaft should stop it's slow rotation, which is what is basically driving the reverse gears (counter-) shaft.