Well, like I said I know its not 100% accurate but close enough for me. And I prefer the slick uppers, so that's what I used on this one. And from what I have read the 610/XM177/GAU-5A was a slick upper while the 609/XM177E1 had a forward assist.

I basically used this image from another site to model mine after (less the half-fence early lower receiver).



As I was never in the military, especially during the time these were issued, I find the various designations given to them by different branches confusing unless I have some reference material handy. Its easier for me to remember them by their original model names, mine is supposed to represent a 610, I suppose I should have just said that at the start. XM177 or GAU-5A are other designations for the same from what I've read.