I favour your explanation. It's not really that important as I'm not likely to need to physically locate the grave. The location as written just raised my curiosity and made me try to make sense of what was meant by 'aces'. The allusion to the playing card was the best my feeble brain could come up with. Yours makes far more sense. HerMaj's comment about grassed areas also makes sense as such memorials could be placed on what would normally be empty grassed walkways between rows (avenues) of graves.