Some of the changes, between a Type 4 and a Type 20..
2 Stanley 45s, old vs new.jpg
Locking bolts....
2 Stanley 45s, Type 4.jpg
And a lot fancier details on the T-4...slotted Brass bolts..vs..
2 Stanley 45s, new sliding stock.jpg
"Plain Jane" details...otherwise they seem to be about the same..

That Type 4 was nickel Plated by Eric Brown, BTW...may have to send the type 20 ( Roxton Pond, QUE. CAN) down to him...looking a bit scruffy..