I think the stock is for sale on the website, I love the retractable so I never bought one.
Here's what I will about mine (HK MP5 SD .22)....it is so much better than I even dreamed it would be and that's not just idle praise. The gun is very well built and so much better than the GSG version all you have to do is pick each one up, look at it...question answered. It's tons of fun to shoot, every time I take mine to the range people always approach me to ask about it and it's accurate. Ok, it's extremely accurate, without a doubt the most accurate .22 I've ever shot with the exception of my bolt guns and it's probably every bit as accurate as they are. I mounted a Vortex Strikefire red/green dot scope on mine and it is pure awesomesauce! With the doubler on the Strikefire it would make a deadly squirrel hunting gun, you could easily make headshots all day long.
The last time I brought her to the range she gathered a lot of attention, mostly people who thought it was a real 9mm MP5 lol, they were pretty amazed how much it looks and feels like the real deal. There was one guy who turns out was a collector of .22LR versions of battle guns and had been wanting an MP5 .22 for his collection and saw mine. He at first thought it was an actual 9mm MP5 like the others did but he was even more excited when he found out it was a .22.
He mentioned he had looked at the GSG version but said it felt "toy like" when handled it and was worried he would break it just by holding it. I could tell he really wanted to get a closer look so I handed it to him and he immediately got a big ol' sh!t eating grin on his face. He couldn't believe it was a .22 because it was built like the real ones. I loaded a magazine and told him to try it out, I thought he was going to cry from happiness lol. He went to his lane and fired three shots and returned the target to find all three in the black. By this time he was really smiling and fired several shots slowly and deliberately before finishing the mag off shooting as fast as be could (which just looks so cool with this gun). He pressed the control switch and brought the target back...he had put all but two shots right in the middle black. Now this is only a 30 meter lane but that was very impressive shooting a gun he had no experience with unsupported and with only a 2X red dot optic. Needless to say he was floored and so enamored with the gun that he passed on shooting a second mag because he wanted to leave immediately and go search the local gun shops for his own.
I don't know if he found one that night, they were very hard to find then and even today I've yet to find another in any of the shops I frequent. I was lucky to get mine since it was the only one they had at the time. I never buy two of the same gun, with the exception of single action revolvers, but if I ever come across another SD I'm definitely buying it! I wanna have one in both hands and go all Rambo, or Rambette, on bunch of water filled bottles lol.
It has definitely been one of the gun purchases I've made that really gave me perma-grin, I truly love the gun and definitely one that will never leave my collection. If anyone is on the fence about buying one of these Walther/HK MP5 .22's run, don't walk, and get it before someone else does. Stay away from those GSG copies or you will hate yourself soon afterwards.
Here's my baby...HK MP5 SD .22 - scary accurate, super reliable, high quality build and more fun than flogging frogs! (not a great picture - was taken more for insurance use.