I did provide an official Soviet document stating they received a T-34-88, a T-34 with an 88 mm cannon, and an official German document stating that they installed 85/88 mm Flak (r) guns in captured Soviet AFVs in my original post.
After a further investigation, it seems not everything stated in the telegram happened on 30th or 31st March 1945.
For the “Commander-in-Chief, Colonel General von Vietinghoff, after a briefing by the anti-tank staff officer (Stopak), ordered that the installation be carried out as quickly as possible.” part, it must have taken place before the 10th March 1945, as Colonel General Heinrich von Vietinghoff was reassigned to command Army Group C on 10th March 1945.
For “The army then had the installation and firing trials carried out by Tank Workshop Company 926 in Libau, and these were successfully completed. The remaining guns are on their way to Libau.” part, according to another telegram named 'FS Abgabe von Geschützen aus Festung Windau” , the remaining guns may have in the process of sending to Libau around 20th March 1945, as the 85/88 mm Flak(r) guns had already been handed over to Tank Workshop Company 926.
The 20th March 1945 telegram referred to another telegram, reference number 410/45. We can further narrow its sending date by comparing the reference numbers and sending dates of other telegrams. It can be deduced that the 410/45 telegram was sent between 8th February 1945 (Nr. 361/45) and 18th February 1945 (Nr. 458/45).
Colonel General von Vietinghoff most likely issued his order via telegram Nr. 410/45 sent between 8th February 1945 and 18th February 1945, or even via a prior one sent on an earlier date.
If this theory is true, the first conversion would have taken place much earlier than the 30th or 31st March 1945. In that case, the first conversion could happen before the formation of A.Pz. Abt . 16 (Beute).
In that case, SU-88 would be the first AFV to be converted, as the T-34s were only listed as operation ready in the 28th March 1945 Deployed Units Overview Table, while the two Beute-StuGs in A.Pz.Abt.16 (Beute) were already as operation ready in the 5 March 1945 Table and very likely took part in the Sixth Battle of Courland in March and was mission-killed in combat.
Otherwise (if everything besides Colonel General von Vietinghoff’s issued order happened no earlier than 31st March 1945), SU-88 would take part in the Seventh Battle of Courland Pocket in April and was mission-killed in combat.
Skimming through the HD scans of those two newsreels again. It seems that the Tactical Numbers of the alleged SU-88 and T-34-88 were 511 and 512, respectively.
No Belgian interwar and cold war boxes. No French pre-war super obscure prototypes. No 44M Tas, no Romanian tank destroyers… who knows what else is missing. Schofield tank… artillery for every nation (SU-14, G.W. Vickers Mk.6/Wespe/Hummel, M12, Bishop) replaced with modern slop…