I actually misread your post and I thought you said it was an L4A4. As an L4A3 the lower is fine, my A3 has a Mk2 lower after FTR (it’s also a Mk2/1 body). The Mk3 butt is quite common and I dare say Kev Groom may have a NOS one with all the hardware.

You can also bar out the lower serial and electro pencil the A3 serial on the left as they did later.