Monitor Audio Silver RX6 Spike Thread size?

Take a trip to Home Depot or a hardware store. I'd bet they are either 1/4" or M6 (metric). 1/4" has a fine and coarse thread available- get both. Just buy one of each size, and maybe one size larger and one size smaller.
See which screw goes in without forcing it, and voila- you'll know the size. Should not be more than 2.00 total, then return the screws and it's free.

The majority of those threads are 1/4-20 or some metric near equivalent. The suggestion to get different screws and try them in the holes is good. The "dash 20" or dash with another number is how many threads per inch there are. A 1/4-20 screw or threaded insert has 20 threads per inch.

monitor audio originates in the UK, so the thread will be metric.

Thanks to all who responded! Went to the hardware this morning with the footer and found that it is actually a M8. I appreciate the suggestions...