Hi Andrew, regarding the rates on these covers, you have three components, but two variables are uncertain:
(a) registration rate
(b) letter rate - depends on weight
(c) compensation rate - depends on how much was paid for.
The Compensation rate seems to be 3/3d on both. This makes sense as it is the maximum compensation available, and these envelopes contained bankbooks or probably money so the maximum was elected.
There might be something funny going on with the registration rate. Ordinarily, the standard registration fee would be added. However, it is possible it was included in the Compensation fee (possibly by mistake?). Or the P.O. might have assumed that by the use of the printed Registration Envelope that no extra was needed.
On the second example, posted paid was 1d + 2s + 1/7d = 3/8d. This might be broken down into Compensation Fee 3/3d + 5d (inland post to 1 oz.) = 3/8d.
On the first example, posted paid was 9d + 1/6d + 1/7d = 3/10d. This might be broken down into Compensation Fee 3/3d + 8d (inland post to 2 oz.) = 3/11d. underpaid
The problem is, even if the lowest Compensation rate was elected (2s), with the 2s Reg fee added, the rates do not work.
Another way of looking at it is:
no extra reg fee required [see back of envelope].
the envelope includes 1/3d for basic compensation up to £5. Compensation to £50 requires 3/3d, thus an extra 2s in stamps.
(c) Inland Postage:
everything over 2s is inland postage, dependent upon weight.
There may of course be other interpretations.