Algeria to Kent with railway post markings

I wonder if anyone could help me with this cover? I am new to collecting postal history and there seems to be an abundance of information on this envelope!
I think the registration marks show the weight, number of seals, and the signature of the postmaster? On the back, however, are some postmarks that seem to indicate carriage by train, via Paris? I would welcome any insights into any of these markings as I am sure I am missing the significance of some!

The boxed "grille de chargement" shows weight, number of seasl, colour of seas and, in the last box, what is written on the seals, in this case "villa parnel" - not sure of spelling of the last word. Sent from Alger the letter crossed to Marseille and by rail to paris. The TPO mark of Marseille - Lyon special is a standard one. The letter passes thro' Paris the next day and then by rail to Calais and in to the UK. All quite straight forward.
Now we come to the franking of 60c and the red "pd" mark. Mu understanding is that the foreign letter rate in 1874 was 30c to which would have been added the registration fee of 60c making 90c in all. The french post office were always very careful with registered items that had to be prepaid so there is a problem here .
The UK post office would not query a rate unless there was some mark applied in France to alert them to insufficient payment.. Then there is, what looks like, a blue "10" which I do not understand..French tax marks are almost always in black and it is difficult to reconcile 10 with the missing amount.You need to have a careful look at the space to the left of the two 30c stamps. Could there have been another stamp there.?
Peter Kelly.

The rate is correct for a letter up to 10 grams at the tariff of 1 July 1870. Registered to GB was 60c and non registered 30c. There is no indication by either the French or British post offices that there was an underpayment. The blue mark may not be postal, it does not look like the figures used by either administration

Chris is quite right about the rate. I had muddled it into something else. Apologies.
Peter Kelly.

Thank you to all who provided such insight. This has been very helpful. I am told by a GB collector that the mysterious blue mark is associated with the Dover post office, but I am still trying to confirm this.
Roger Niven