Ken G - the subframe you have is from the 122/3 series and 70/100 I believe. This was before the "Quick Attach Latch" came along (which I believe was at the time of your 124. To use this frame I believe all you need to do is remove those short frame extenders and set them aside (but don't discard them - I'll tell you why later). Then come up with a rod long enough to slide thru the top holes in the sub-frame arms (the vertical pieces that are welded to the sub-frame). You can drill a couple holes in the rod on the area outside those sub-frame arms, and use washers and cotter pins to hold it in place. Then SHAZAMMM - you can use your subframe with your Quick Attach Latch.
Now, on those frame extenders - fifteen years ago or so, I had a 122 with that same sub-frame BUT no frame extenders. At that time I didn't know what they even looked like. I visited my trusty local CC dealer and sure'nuf he did NOT stock them, so I ordered me up a pair. About a week later I got the call they were in and beat feet to the dealership. Low and Behold I was handed 2 really nice pieces of what appeared to be pieces of flat stock with a couple of holes and with one end rounded. THEY WERE PAINTED RED and had an IH sticker part number tag on them. At that time I believe I paid $11 apiece. I toyed with the idea of making up a pair and keeping the read ones with the IH tag, but finally decided, they probably came out of the IH parts bin used for something else besides the CC frame extenders. Never did find out. BUT, I suspect you might just find some other guys out there in a situation reverse of yours and like mine. Their sub-frame is missing the extenders and they don't know how to attach it to their 122 because it doesn't go up high enough. Oh, well, maybe they'll catch this post.
Let us know if you're able to do what I suggested. I'm certain it will work.
Hydro Harry
Old Cubs Never Die (but occassionally have mixed up parts)