Some thoughts on this:
I believe the lumps on those DRs went through the bottom of the action because of the larger hinge pins H&H used to withstand the extreme pressures and energy those guns generated.
The way it was explained to me was that a bigger pin means a bigger lump which means more surface area to displace the energy generated by these rifles. Without these enlarged surfaces, these rifles would have gone off the face very quickly.
If the maker wanted to keep the lumps enclosed, they would have been forced to use a bigger action. This would have increased the gun's weight and ruined some of its sleek, aesthetic appeal.
No, that didn't have anything to do with it. Had nothing to do with early vs late either.
Here's a .577/100/750 NE Royal, finished rather plain, without thru-lump:
http://www.pugsguns.com/findItem.action?id=1734Here's a rather early .500/.450 NE Royal without thru-lump:
http://www.champlinarms.com/Default.aspx...&GunID=1386Here's a very early .500/.465 NE Royal, without thru-lump:
http://www.champlinarms.com/Default.aspx...&GunID=1520Of course, the .577/100/750 NE and .500/.465 NE are quite low pressure - they're both 14 tons BaseCUP. The .500/.450 is somewhat higher at 15.5 tons. The above rifles were all built without through lumps, but Holland built others in the same calibers to the same weights with through lumps.
If high pressure is the reason for the through lumps, we'd sure see more of them in the much higher pressure calibers.
Here's a .425 Westley Richards Rimless NE (18.5 tons) Royal from the late '20s without thru-lump:
http://www.pugsguns.com/findItem.action?id=1736Here's a .275 H & H Magnum (18 tons) Royal from 1926 without thru-lump:
http://www.pugsguns.com/findItem.action?id=1824Through the spectrum of calibers, from the low pressure .577, .465, and .375 2.5" Nitro Expresses to the high pressure/high velocity magnums, you'll find Holland Royal and Modele DeLuxe double rifles built both ways, even though they're otherwise identical rifles. Holland was always particular about strength in their double rifles. After the emergence of Cordite and high pressure cartridges, other gunmakers like Purdey and Rigby continued to build DRs on bar-actions, while Holland insisted on back-actions because they have more steel in the action bar at the break-off. Had the thru-lump issue had anything to do with pressure or strength, I sincerely doubt that Holland would have used both methods interchangeably, as they clearly did.