[sportruck]

I found this code in include/product_modify.php

if (empty($product_info)) {
	$sku_prefix = 'SKU';
	$product_info['productcode'] = array_pop(array_pop(func_query("SELECT MAX(productid) FROM $sql_tbl[products]")));
	$plus = 0;
	while(func_query_first_cell("SELECT COUNT(*) FROM $sql_tbl[products] WHERE productcode='$sku_prefix".($product_info['productcode'].(++$plus))."'")) {
	}
	$product_info['productcode'] = $sku_prefix.($product_info['productcode']+$plus);

I'm not sure why that is in there... but it could be where your prefix is coming from.

Update: It looks like when you add a new product it defaults the SKU to be the productid prepended by SKU. You should be able to change it as you fill out the item info.