From the part number AL84T (and mention of "Prime", which was a predecessor to Ameritron) in the Dahl catalog pages that were uploaded and cited here, your transformer appears to be a replacement transformer for the Ameritron Al84 (four sweep tube, HF) amplifier.
Sweep tubes in this sort of amplifier often run about 1kv on the plate (others here can almost certainly clarify/correct.)
A quick perusal of some 7289 (2C39) type 23cm PA projects seems to recommend 1200-1400VDC on the plate(s), so your transformer might yield at the low end of that range if you use the higher-voltage secondary tap (850 VAC * 1.4).
Best of luck on your project and welcome to 1296!
