Censored Spanish Cover To Germany My Dear Bertie.. Re A Serious Situation? U Number?

I really like this one. I am going to say I feel anxious about whether Bertie has actually sent thanks to Ernest regarding their 'serious' situation which is rather intriguing. ALso Tarjeta Postal = postcard? ?what is that U number above it? Any information welcome.

This? May explain why it's among the moas

http://www.heymannfamily.com/other%...0Photos.html

This is an uprated piece of postal stationery used during the Spanish Civil war on the Republican (Government) side. I am not clear on the year, could be dated 1936 or 1938 but then the cancel seems to be from 1937 - at least the last digit looks like a 7.

All these postcards had a control number, that is the U #### above the "TARJETA POSTAL" which means postcard. 15c was the domestic rate prior to the start of the war so it needed additional postage for a foreign destination.

I think I see 24SEP36.7A (7AM). So, from the relatively early days of the civil war. And clearly, it was wartime with the CENSURADA censor marking.