*** Updated by M. Eberhard 21 Jan 2024

I think you'll find a 380 and a 384 are not the same :-)  But the 8270/8271
substitutions by 74178/74179 seem to match and are good to know.

A leading additional 5 or 9 means DEC selected examples of devices, eg 
97401/7401, 380/5380, etc against some particular criteria, such as 
leakage, or drive current.

Here's a list I found some time ago (most of the comments are not mine, 
and I'm sorry I've lost the attribution, but I've updated it slightly):

<Updated my M. Eberhard...>

DEC component cross reference / substitutions
     DEC                EIE
D-003, D003            1N994
D-662, D662            1N645
D-664, D664            1N3606
D-668, D668            Two 1N3606 in series
D-670, D670            1N3653
D-671, D671            1N3653
D-672, D672            1N3653
DEC 999, DEC999        MM999
DEC 1008, DEC1008      MM1008
DEC 2219, DEC2219      2N2219
DEC 2501               FSA2501 Diode Array
2N2904                 2N2118A
DEC 2904, DEC2904      2N1132
DEC 3009, DEC3009      2N3009
DEC 3009A, DEC3009A    2N3009
DEC 3009B, DEC3009B    2N3009
DEC 3494               2N3494
DEC 3500, DEC3500      2N3500
DEC 3568, DEC3568      2N3568
DEC 3634, DEC3634      2N3634
DEC 3638, DEC3638      2N3638
DEC 3639, DEC3639      2N3639
DEC 3639B, DEC3639B    2N3639
DEC 3639-0             2N3639
DEC 3639-2, DEC3639-2  2N3639-2
DEC 3715, DEC3715      2N3715
DEC 3790, DEC3790      2N3790
DEC 4258, DEC4258      2N4258
DEC 6531, DEC6531      MPS6531
DEC 6534, DEC6534      MPS6534
SDA-6, SDA6            2N2060
1N429                  1N429 6.2V 5%
1N449                  1N449 6.2V 5%
1N762                  1N762 5.5V 250 ohms 5%
1/4M6.8AZ5             1N4099
1/4M2.4VAZ5            1N4370
MR2064                 1N4001
MR2066                 1N4003
U6A900959X             7440
MPSA05                 GPSA05 (Pinout reversed)
MPSA55                 GPSA55 (Pinout reversed)

Connector
1209456                AMP 61320-1
1209340                Tyco/AMP 1-480459-0
 From PDP-8 maintenance manual 10-5 and RK05 prints.

Also I have found the following parts to be similar but this is not from
DEC documentation so check your particular part.
DEC 82xx is Signetics 82xx series
DEC 5xxx is Signetics SPxxx
DEC 88xx is National DS88xx series or also I think Signetics 8Txx series
DEC 97401 is also labeled a SN7401, was a selected version of that part
     for low leakage current.
DEC 314, DEC314		SP314A
DEC 380, DEC380		SP380A, also DS8640 is similar
DEC 384, DEC384 DEC5384	SP384A
DEC 8881, DEC8881	SN7401 has same pinout but less drive, SN7439
                         has the drive and same pinout.

For R series repairs B Klatt said:
I've successfully used 1N4148-type diodes and 2N3904 (NPN) and
2N3906 (PNP) as general purpose substitutes in the few R-series
repairs I've made.   The MPSA05 and MPSA55 were popular with
DEC designers and are still readily available.

-- 
Pete

Pete Turnbull