If you still have RouterOS in place, you can look for the oldest firmware your hardware can fallback to by : using Winbox, navigate to System > RouterBOARD
Example for RB4011 model :
Although it is a setting for U-BOOT (ie. bootloader, not the running OS), the running OS firmware could not predate the Factory Firmware version listed in the box above. In this example, you cannot install any version earlier than 6.43 even if you wish.
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I believe RB5009 has a factory firmware of 7.0.x since release and there was no factory firmware in 6.x series for this product. See also this thread in Mikrotik forum.