Bermuda First Day Cover 1925

I picked up at auction this cover and two others that flew on the airship Los Angeles from Bermuda to New York in 1925. I have two questions on this item:

First, this cover has a "First Day Cover" auxiliary mark. The block of four is Bermuda Scott 48a. That stamp was issued in 1925, but I have not been able to find the month and date of issue. The postmark date is April 13, 1925. Was that the date of issue?

Second, why was this cover divided into quadrants with one registry mark in each quadrant? At first I thought one mark for each stamp, but that doesn't make sense because there are two NYC, NY and two Milburn, NJ registry marks. I'm sure Edward Worden, a well known FDC pioneer, had his reasons.

Any insight would be appreciated.

Regards,
Capt Hickey

The cross mark is typically found on most Registered covers across the world. Have you checked the 'firsts' for the Los Angles flights?
Don

Thanks for the lead on the blue cross. I didn't realize that was standard practice. Here's an explanation I found on Wikipedia:

"The registration of letters as known today was introduced in 1841 in Great Britain. The letter had to be enclosed within a large sheet of green paper. The green sheet was addressed to the Post Office where the recipient lived. The green sheet was then used as a receipt and was returned to the office of origin after delivery. On 1 July 1858 the green sheet was replaced by a green silk ribbon and shortly afterwards by a green linen tape. In 1870 the tape was replaced by green string. On the introduction of postal stationery registration envelopes in 1878 the string was replaced by printed blue crossed lines. The blue crossed lines have survived on registered letters to the present day."

I'll recheck the listing of Los Angeles covers.

Thanks again.