No. 4 Mk I* bubba'd into a carbine. The butt stock is the same and may be a real No. 5 Rifle(aka Jungle Carbine) stock. Same mag in a No. 5 as a No. 4.
Gunparts currently has No. 4(they list No. 4 parts under 'SMLE' for some reason. Been yelling at 'em for more than 20 years that a No. 4 is not an SMLE) mags on sale at $39.95. Regular price is $54.75. They list .303 Brit charger clips at $2.75 each too.
No. 1 Mk III mags do not fit in a No. 4 unless the bottom locking lug is ground off. Difference between the mags is the No. 1 mag has 2 locking lugs while a No. 4 mag has 1.
Sarco wants $30 for a repro mag. $3 for a charger clip.