Gents, I believe that you need a "clipped and pinned" SEF G3 lower to work on your PTR-91. I don't think a stock German surplus SEF lower will fit, and if it did it may be a touch felonious.
Check out Robert Johnson's site for all your HK needs: (the following is from him)
HK SEF plastic G3 lowers are full auto and will not fit onto a semi auto receiver. You can legally convert these lowers to fit onto a semi receiver using a semi fire control group. This conversion is called a 'clip and pin'. We now offer New HK Plastic SEF navy style lowers clipped & pinned with selector for $125. A professional HK gun smith has done the clipped & pin conversions and the work is top rate. Once converted the lower accepts your existing semi auto fire control group and mounts onto your semi auto receiver. This gives you the look of a real full auto while maintaining a legal semi auto rifle. HK G3 Clipped & Pinned lowers fit on any HK91 type rifle including PTR91, Vector V51, Century G3 clones and all other G3 style rifles. Picture below. More pictures and information here:
http://www.robertrtg.com/g3sefclippin.html
Also check out Bill Springfield's site, he has what you want:
http://www.triggerwork.net/hkpacks.html
Check out Robert Johnson's site for all your HK needs: (the following is from him)
HK SEF plastic G3 lowers are full auto and will not fit onto a semi auto receiver. You can legally convert these lowers to fit onto a semi receiver using a semi fire control group. This conversion is called a 'clip and pin'. We now offer New HK Plastic SEF navy style lowers clipped & pinned with selector for $125. A professional HK gun smith has done the clipped & pin conversions and the work is top rate. Once converted the lower accepts your existing semi auto fire control group and mounts onto your semi auto receiver. This gives you the look of a real full auto while maintaining a legal semi auto rifle. HK G3 Clipped & Pinned lowers fit on any HK91 type rifle including PTR91, Vector V51, Century G3 clones and all other G3 style rifles. Picture below. More pictures and information here:
http://www.robertrtg.com/g3sefclippin.html
Also check out Bill Springfield's site, he has what you want:
http://www.triggerwork.net/hkpacks.html