Back around 1975, I got the same vise free because the front jaw was broken right where it says No. 6. Got it welded and it lasted until 2005. Changing a driveshaft U-joint finally did it in.
Checked around for another jaw and one from Record (Irwin) was more than a new vise (with freight etc.) I remembered Princess Auto sold Records, so I checked with a guy there that I kinda knew. He said they didn't fool around with parts because they offered a "Lifetime Guarantee" on those vises. Oh really!! I explained the circumstances....didn't buy it there, welded etc and he said bring it in. Couldn't get home fast enough. My new vise now sits proudly in my shop.
Nice clean up job you are doing, with some paint should look like new again.
If you change both jaws it should correct the height diff. You know yours are the early versions as they have "closed" mounting holes. (Open on my new one. and still made in England)
What's my favorite store?? Hint.PAL